#vulnerable code 
                                private PropertyMapping getConstantMapping(List<MapperReference> mapperReferences,
                                               List<SourceMethod> methods,
                                               SourceMethod method,
                                               String constantExpression,
                                               ExecutableElement targetAccessor,
                                               String dateFormat,
                                               List<TypeMirror> qualifiers) {

        // source
        String mappedElement = "constant '" + constantExpression + "'";
        Type sourceType = typeFactory.getType( String.class );

        // target
        Type targetType = typeFactory.getSingleParameter( targetAccessor ).getType();
        String targetPropertyName = Executables.getPropertyName( targetAccessor );

        Assignment assignment = mappingResolver.getTargetAssignment(
            method,
            mappedElement,
            mapperReferences,
            methods,
            sourceType,
            targetType,
            targetPropertyName,
            dateFormat,
            qualifiers,
            constantExpression
        );

        if ( assignment != null ) {

            // target accessor is setter, so decorate assignment as setter
            assignment = new SetterWrapper( assignment, method.getThrownTypes() );
        }
        else {
            messager.printMessage(
                Kind.ERROR,
                String.format(
                    "Can't map \"%s %s\" to \"%s %s\".",
                    sourceType,
                    constantExpression,
                    targetType,
                    targetPropertyName
                ),
                method.getExecutable()
            );
        }

        return new PropertyMapping( targetAccessor.getSimpleName().toString(), targetType, assignment );
    } 
                            #location 14 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private PropertyMapping getConstantMapping(List<MapperReference> mapperReferences,
                                               List<SourceMethod> methods,
                                               SourceMethod method,
                                               String constantExpression,
                                               ExecutableElement targetAccessor,
                                               String dateFormat,
                                               List<TypeMirror> qualifiers) {

        // source
        String mappedElement = "constant '" + constantExpression + "'";
        Type sourceType = typeFactory.getType( String.class );

        // target
        Type targetType;
        if ( Executables.isSetterMethod( targetAccessor ) ) {
            targetType = typeFactory.getSingleParameter( targetAccessor ).getType();
        }
        else {
            targetType = typeFactory.getReturnType( targetAccessor );
        }
        String targetPropertyName = Executables.getPropertyName( targetAccessor );

        Assignment assignment = mappingResolver.getTargetAssignment(
            method,
            mappedElement,
            mapperReferences,
            methods,
            sourceType,
            targetType,
            targetPropertyName,
            dateFormat,
            qualifiers,
            constantExpression
        );

        if ( assignment != null ) {

            // target accessor is setter, so decorate assignment as setter
            assignment = new SetterWrapper( assignment, method.getThrownTypes() );

            // wrap when dealing with getter only on target
            if ( Executables.isGetterMethod( targetAccessor ) ) {
                assignment = new GetterCollectionOrMapWrapper( assignment );
            }
        }
        else {
            messager.printMessage(
                Kind.ERROR,
                String.format(
                    "Can't map \"%s %s\" to \"%s %s\".",
                    sourceType,
                    constantExpression,
                    targetType,
                    targetPropertyName
                ),
                method.getExecutable()
            );
        }

        return new PropertyMapping( targetAccessor.getSimpleName().toString(), targetType, assignment );
    }