#vulnerable code 
                              @Override
  public boolean isEquivalentTo(JSType otherType) {
    if (!(otherType instanceof FunctionType)) {
      return false;
    }
    FunctionType that = (FunctionType) otherType;
    if (!that.isFunctionType()) {
      return false;
    }
    if (this.isConstructor()) {
      if (that.isConstructor()) {
        return this == that;
      }
      return false;
    }
    if (this.isInterface()) {
      if (that.isInterface()) {
        return this.getReferenceName().equals(that.getReferenceName());
      }
      return false;
    }
    if (that.isInterface()) {
      return false;
    }
    return this.typeOfThis.isEquivalentTo(that.typeOfThis) &&
        this.call.isEquivalentTo(that.call);
  } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public boolean isEquivalentTo(JSType otherType) {
    FunctionType that =
        JSType.toMaybeFunctionType(otherType.toMaybeFunctionType());
    if (that == null) {
      return false;
    }
    if (this.isConstructor()) {
      if (that.isConstructor()) {
        return this == that;
      }
      return false;
    }
    if (this.isInterface()) {
      if (that.isInterface()) {
        return this.getReferenceName().equals(that.getReferenceName());
      }
      return false;
    }
    if (that.isInterface()) {
      return false;
    }
    return this.typeOfThis.isEquivalentTo(that.typeOfThis) &&
        this.call.isEquivalentTo(that.call);
  }