output: #fixed code
                                private FlowScope traverse(Node n, FlowScope scope) {
      switch (n.getType()) {
        case Token.ASSIGN:
          scope = traverseAssign(n, scope);
          break;
  
        case Token.NAME:
          scope = traverseName(n, scope);
          break;
  
        case Token.GETPROP:
          scope = traverseGetProp(n, scope);
          break;
  
        case Token.AND:
          scope = traverseAnd(n, scope).getJoinedFlowScope()
              .createChildFlowScope();
          break;
  
        case Token.OR:
          scope = traverseOr(n, scope).getJoinedFlowScope()
              .createChildFlowScope();
          break;
  
        case Token.HOOK:
          scope = traverseHook(n, scope);
          break;
  
        case Token.OBJECTLIT:
          scope = traverseObjectLiteral(n, scope);
          break;
  
        case Token.CALL:
          scope = traverseCall(n, scope);
          break;
  
        case Token.NEW:
          scope = traverseNew(n, scope);
          break;
  
        case Token.ASSIGN_ADD:
        case Token.ADD:
          scope = traverseAdd(n, scope);
          break;
  
        case Token.POS:
        case Token.NEG:
          scope = traverse(n.getFirstChild(), scope);  // Find types.
          n.setJSType(getNativeType(NUMBER_TYPE));
          break;
  
        case Token.ARRAYLIT:
          scope = traverseArrayLiteral(n, scope);
          break;
  
        case Token.THIS:
          n.setJSType(scope.getTypeOfThis());
          break;
  
        case Token.ASSIGN_LSH:
        case Token.ASSIGN_RSH:
        case Token.LSH:
        case Token.RSH:
        case Token.ASSIGN_URSH:
        case Token.URSH:
        case Token.ASSIGN_DIV:
        case Token.ASSIGN_MOD:
        case Token.ASSIGN_BITAND:
        case Token.ASSIGN_BITXOR:
        case Token.ASSIGN_BITOR:
        case Token.ASSIGN_MUL:
        case Token.ASSIGN_SUB:
        case Token.DIV:
        case Token.MOD:
        case Token.BITAND:
        case Token.BITXOR:
        case Token.BITOR:
        case Token.MUL:
        case Token.SUB:
        case Token.DEC:
        case Token.INC:
        case Token.BITNOT:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(NUMBER_TYPE));
          break;
  
        case Token.PARAM_LIST:
          scope = traverse(n.getFirstChild(), scope);
          n.setJSType(getJSType(n.getFirstChild()));
          break;
  
        case Token.COMMA:
          scope = traverseChildren(n, scope);
          n.setJSType(getJSType(n.getLastChild()));
          break;
  
        case Token.TYPEOF:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(STRING_TYPE));
          break;
  
        case Token.DELPROP:
        case Token.LT:
        case Token.LE:
        case Token.GT:
        case Token.GE:
        case Token.NOT:
        case Token.EQ:
        case Token.NE:
        case Token.SHEQ:
        case Token.SHNE:
        case Token.INSTANCEOF:
        case Token.IN:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(BOOLEAN_TYPE));
          break;
  
        case Token.GETELEM:
          scope = traverseGetElem(n, scope);
          break;
  
        case Token.EXPR_RESULT:
          scope = traverseChildren(n, scope);
          if (n.getFirstChild().isGetProp()) {
            ensurePropertyDeclared(n.getFirstChild());
          }
          break;
  
        case Token.SWITCH:
          scope = traverse(n.getFirstChild(), scope);
          break;
  
        case Token.RETURN:
          scope = traverseReturn(n, scope);
          break;
  
        case Token.VAR:
        case Token.THROW:
          scope = traverseChildren(n, scope);
          break;
  
        case Token.CATCH:
          scope = traverseCatch(n, scope);
          break;
  
        case Token.CAST:
          scope = traverseChildren(n, scope);
          JSDocInfo info = n.getJSDocInfo();
          if (info != null && info.hasType()) {
            n.setJSType(info.getType().evaluate(syntacticScope, registry));
          }
          break;
      }
  
      return scope;
    }
input: #vulnerable code 
                                private FlowScope traverse(Node n, FlowScope scope) {
      switch (n.getType()) {
        case Token.ASSIGN:
          scope = traverseAssign(n, scope);
          break;
  
        case Token.NAME:
          scope = traverseName(n, scope);
          break;
  
        case Token.GETPROP:
          scope = traverseGetProp(n, scope);
          break;
  
        case Token.AND:
          scope = traverseAnd(n, scope).getJoinedFlowScope()
              .createChildFlowScope();
          break;
  
        case Token.OR:
          scope = traverseOr(n, scope).getJoinedFlowScope()
              .createChildFlowScope();
          break;
  
        case Token.HOOK:
          scope = traverseHook(n, scope);
          break;
  
        case Token.OBJECTLIT:
          scope = traverseObjectLiteral(n, scope);
          break;
  
        case Token.CALL:
          scope = traverseCall(n, scope);
          break;
  
        case Token.NEW:
          scope = traverseNew(n, scope);
          break;
  
        case Token.ASSIGN_ADD:
        case Token.ADD:
          scope = traverseAdd(n, scope);
          break;
  
        case Token.POS:
        case Token.NEG:
          scope = traverse(n.getFirstChild(), scope);  // Find types.
          n.setJSType(getNativeType(NUMBER_TYPE));
          break;
  
        case Token.ARRAYLIT:
          scope = traverseArrayLiteral(n, scope);
          break;
  
        case Token.THIS:
          n.setJSType(scope.getTypeOfThis());
          break;
  
        case Token.ASSIGN_LSH:
        case Token.ASSIGN_RSH:
        case Token.LSH:
        case Token.RSH:
        case Token.ASSIGN_URSH:
        case Token.URSH:
        case Token.ASSIGN_DIV:
        case Token.ASSIGN_MOD:
        case Token.ASSIGN_BITAND:
        case Token.ASSIGN_BITXOR:
        case Token.ASSIGN_BITOR:
        case Token.ASSIGN_MUL:
        case Token.ASSIGN_SUB:
        case Token.DIV:
        case Token.MOD:
        case Token.BITAND:
        case Token.BITXOR:
        case Token.BITOR:
        case Token.MUL:
        case Token.SUB:
        case Token.DEC:
        case Token.INC:
        case Token.BITNOT:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(NUMBER_TYPE));
          break;
  
        case Token.PARAM_LIST:
          scope = traverse(n.getFirstChild(), scope);
          n.setJSType(getJSType(n.getFirstChild()));
          break;
  
        case Token.COMMA:
          scope = traverseChildren(n, scope);
          n.setJSType(getJSType(n.getLastChild()));
          break;
  
        case Token.TYPEOF:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(STRING_TYPE));
          break;
  
        case Token.DELPROP:
        case Token.LT:
        case Token.LE:
        case Token.GT:
        case Token.GE:
        case Token.NOT:
        case Token.EQ:
        case Token.NE:
        case Token.SHEQ:
        case Token.SHNE:
        case Token.INSTANCEOF:
        case Token.IN:
          scope = traverseChildren(n, scope);
          n.setJSType(getNativeType(BOOLEAN_TYPE));
          break;
  
        case Token.GETELEM:
          scope = traverseGetElem(n, scope);
          break;
  
        case Token.EXPR_RESULT:
          scope = traverseChildren(n, scope);
          if (n.getFirstChild().isGetProp()) {
            ensurePropertyDeclared(n.getFirstChild());
          }
          break;
  
        case Token.SWITCH:
          scope = traverse(n.getFirstChild(), scope);
          break;
  
        case Token.RETURN:
          scope = traverseReturn(n, scope);
          break;
  
        case Token.VAR:
        case Token.THROW:
          scope = traverseChildren(n, scope);
          break;
  
        case Token.CATCH:
          scope = traverseCatch(n, scope);
          break;
  
        case Token.CAST:
          scope = traverseChildren(n, scope);
          break;
      }
  
      // TODO(johnlenz): remove this after the CAST node change has shaken out.
      if (!n.isFunction()) {
        JSDocInfo info = n.getJSDocInfo();
        if (info != null && info.hasType()) {
          JSType castType = info.getType().evaluate(syntacticScope, registry);
  
          // A stubbed type declaration on a qualified name should take
          // effect for all subsequent accesses of that name,
          // so treat it the same as an assign to that name.
          if (n.isQualifiedName() &&
              n.getParent().isExprResult()) {
            updateScopeForTypeChange(scope, n, n.getJSType(), castType);
          }
  
          n.setJSType(castType);
        }
      }
  
      return scope;
    } 
                              #location 155 
                              #vulnerability type NULL_DEREFERENCE