#vulnerable code 
                              private EnvTypePair analyzeLooseCallNodeBwd(
      Node callNode, TypeEnv outEnv, JSType retType) {
    Preconditions.checkArgument(callNode.isCall());
    Preconditions.checkNotNull(retType);
    Node callee = callNode.getFirstChild();
    TypeEnv tmpEnv = outEnv;
    FunctionTypeBuilder builder = new FunctionTypeBuilder();
    for (int i = callNode.getChildCount() - 2; i >= 0; i--) {
      Node arg = callNode.getChildAtIndex(i + 1);
      tmpEnv = analyzeExprBwd(arg, tmpEnv).env;
      // Wait until FWD to get more precise argument types.
      builder.addReqFormal(JSType.BOTTOM);
    }
    JSType looseRetType = retType.isUnknown() ? JSType.BOTTOM : retType;
    JSType looseFunctionType =
        builder.addRetType(looseRetType).addLoose().buildType();
    looseFunctionType.getFunType().checkValid();
    println("loose function type is ", looseFunctionType);
    EnvTypePair calleePair = analyzeExprBwd(callee, tmpEnv, looseFunctionType);
    return new EnvTypePair(calleePair.env, retType);
  } 
                            #location 17 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              private EnvTypePair analyzeLooseCallNodeBwd(
      Node callNode, TypeEnv outEnv, JSType retType) {
    Preconditions.checkArgument(callNode.isCall());
    Preconditions.checkNotNull(retType);
    Node callee = callNode.getFirstChild();
    TypeEnv tmpEnv = outEnv;
    FunctionTypeBuilder builder = new FunctionTypeBuilder();
    for (int i = callNode.getChildCount() - 2; i >= 0; i--) {
      Node arg = callNode.getChildAtIndex(i + 1);
      tmpEnv = analyzeExprBwd(arg, tmpEnv).env;
      // Wait until FWD to get more precise argument types.
      builder.addReqFormal(JSType.BOTTOM);
    }
    JSType looseRetType = retType.isUnknown() ? JSType.BOTTOM : retType;
    JSType looseFunctionType =
        builder.addRetType(looseRetType).addLoose().buildType();
    println("loose function type is ", looseFunctionType);
    EnvTypePair calleePair = analyzeExprBwd(callee, tmpEnv, looseFunctionType);
    return new EnvTypePair(calleePair.env, retType);
  }