#vulnerable code 
                              FunctionTypeBuilder inferThisType(JSDocInfo info,
      @Nullable Node owner) {
    ObjectType maybeThisType = null;
    if (info != null && info.hasThisType()) {
      maybeThisType = ObjectType.cast(
          info.getThisType().evaluate(scope, typeRegistry));
    }
    if (maybeThisType != null) {
      // TODO(user): Doing an instanceof check here is too
      // restrictive as (Date,Error) is, for instance, an object type
      // even though its implementation is a UnionType. Would need to
      // create interfaces JSType, ObjectType, FunctionType etc and have
      // separate implementation instead of the class hierarchy, so that
      // union types can also be object types, etc.
      thisType = maybeThisType;
    } else if (owner != null &&
               (info == null || !info.hasType())) {
      // If the function is of the form:
      // x.prototype.y = function() {}
      // then we can assume "x" is the @this type. On the other hand,
      // if it's of the form:
      // /** @type {Function} */ x.prototype.y;
      // then we should not give it a @this type.
      String ownerTypeName = owner.getQualifiedName();
      ObjectType ownerType = ObjectType.cast(
          typeRegistry.getType(
              scope, ownerTypeName, sourceName,
              owner.getLineno(), owner.getCharno()));
      if (ownerType != null) {
        thisType = ownerType;
      }
    }

    return this;
  } 
                            #location 26 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              FunctionTypeBuilder(String fnName, AbstractCompiler compiler,
      Node errorRoot, String sourceName, Scope scope) {
    Preconditions.checkNotNull(errorRoot);

    this.fnName = fnName == null ? "" : fnName;
    this.codingConvention = compiler.getCodingConvention();
    this.typeRegistry = compiler.getTypeRegistry();
    this.errorRoot = errorRoot;
    this.sourceName = sourceName;
    this.compiler = compiler;
    this.scope = scope;
  }