output: #fixed code
                                  private List<Method> retrieveMethods(TypeElement element, boolean implementationRequired) {
          List<Method> methods = new ArrayList<Method>();
  
          MapperPrism mapperPrism = implementationRequired ? MapperPrism.getInstanceOn( element ) : null;
  
          for ( ExecutableElement executable : methodsIn( element.getEnclosedElements() ) ) {
              Method method = getMethod( element, executable, implementationRequired );
              if ( method != null ) {
                  methods.add( method );
              }
          }
  
          //Add all methods of used mappers in order to reference them in the aggregated model
          if ( implementationRequired ) {
              for ( TypeMirror usedMapper : mapperPrism.uses() ) {
                  methods.addAll(
                      retrieveMethods(
                          (TypeElement) ( (DeclaredType) usedMapper ).asElement(),
                          false
                      )
                  );
              }
          }
  
          return methods;
      }
input: #vulnerable code 
                                  private List<Method> retrieveMethods(TypeElement element, boolean implementationRequired) {
          List<Method> methods = new ArrayList<Method>();
  
          MapperPrism mapperPrism = implementationRequired ? MapperPrism.getInstanceOn( element ) : null;
  
          //TODO Extract to separate method
          for ( ExecutableElement method : methodsIn( element.getEnclosedElements() ) ) {
              Parameter parameter = executables.retrieveParameter( method );
              Type returnType = executables.retrieveReturnType( method );
  
              boolean mappingErroneous = false;
  
              if ( implementationRequired ) {
                  if ( parameter.getType().isIterableType() && !returnType.isIterableType() ) {
                      printMessage(
                          ReportingPolicy.ERROR,
                          "Can't generate mapping method from iterable type to non-iterable type.",
                          method
                      );
                      mappingErroneous = true;
                  }
                  if ( !parameter.getType().isIterableType() && returnType.isIterableType() ) {
                      printMessage(
                          ReportingPolicy.ERROR,
                          "Can't generate mapping method from non-iterable type to iterable type.",
                          method
                      );
                      mappingErroneous = true;
                  }
                  if ( parameter.getType().isPrimitive() ) {
                      printMessage(
                          ReportingPolicy.ERROR,
                          "Can't generate mapping method with primitive parameter type.",
                          method
                      );
                      mappingErroneous = true;
                  }
                  if ( returnType.isPrimitive() ) {
                      printMessage(
                          ReportingPolicy.ERROR,
                          "Can't generate mapping method with primitive return type.",
                          method
                      );
                      mappingErroneous = true;
                  }
  
                  if ( mappingErroneous ) {
                      continue;
                  }
              }
  
              //add method with property mappings if an implementation needs to be generated
              if ( implementationRequired ) {
                  methods.add(
                      Method.forMethodRequiringImplementation(
                          method,
                          parameter.getName(),
                          parameter.getType(),
                          returnType,
                          getMappings( method )
                      )
                  );
              }
              //otherwise add reference to existing mapper method
              else {
                  methods.add(
                      Method.forReferencedMethod(
                          typeUtil.getType( typeUtils.getDeclaredType( element ) ),
                          method,
                          parameter.getName(),
                          parameter.getType(),
                          returnType
                      )
                  );
              }
          }
  
          //Add all methods of used mappers in order to reference them in the aggregated model
          if ( implementationRequired ) {
              for ( TypeMirror usedMapper : mapperPrism.uses() ) {
                  methods.addAll(
                      retrieveMethods(
                          (TypeElement) ( (DeclaredType) usedMapper ).asElement(),
                          false
                      )
                  );
              }
          }
  
          return methods;
      } 
                              #location 57 
                              #vulnerability type NULL_DEREFERENCE