Issue with fltk callback using this pointer

Question

void MainMenu::cb_userinput(fltk::Input *in, void *v)
{
    MainMenu *mm = (MainMenu*)v;
    memset(user_input, '\0', sizeof(user_input));
    strcpy((char*)user_input, in->value());
    in->value('\0');
    memset(user_input, '\0', sizeof(user_input));
    mm->ExitFromMenu();
}
void MainMenu::ExitFromMenu()
{
    Menu->clear();
    Menu->end();
}

the line I am calling callback

input->callback((fltk::Callback*)cb_userinput, this);

callback function is called successfully, if I remove both the lines from below function, then my program runs successfully but if I use both of these lines then program gives segmentation fault error,

void MainMenu::ExitFromMenu()
{
    //Menu->clear();
    //Menu->end();
}

I can't use gdb as this program is running on remote device, can anyone help me to find out the issue? thanks in advance.

Full class

class MainMenu : public IUserOptions
{
    protected:
        Group *Menu;
        void DisplayUserOptions(); 
        static char user_input[3];
        static void cb_userinput(fltk::Input *in, void *v);
        int user_choice;
        void ExitFromMenu();
    public:
        MainMenu(vector<string> userChoices);
        int GetUserOption();
        ~MainMenu();
};

char MainMenu::user_input[3];
MainMenu::MainMenu(vector<string> userChoices) : IUserOptions(userChoices)
{
    Menu = new fltk::Group(0, 0, 320, 220, "");
}
void MainMenu::cb_userinput(fltk::Input *in, void *v)
{
    MainMenu *mm = (MainMenu*)v;
  memset(user_input, '\0', sizeof(user_input));
  strcpy((char*)user_input, in->value());
    in->value('\0');
    memset(user_input, '\0', sizeof(user_input));
    mm->ExitFromMenu();
}
void MainMenu::ExitFromMenu()
{
    Menu->clear();
    Menu->end();
}
void MainMenu::DisplayUserOptions()
{
    Menu->set_vertical();
    Menu->begin();

    fltk::Widget *Wel = new fltk::Widget(0, 0,320, 220, "");
    Wel->color((fltk::Color)0xf5751000);
    Wel->box(fltk::FLAT_BOX);

    fltk::Widget *MAIN_MENU_BOX = new Widget(0, 0, 320, 30, "Genus Power Vers 1.0.0");
    MAIN_MENU_BOX->box(FLAT_BOX);
    MAIN_MENU_BOX->color((fltk::Color)0x6E040600);
    MAIN_MENU_BOX->labelfont(fltk::TIMES_BOLD);
    MAIN_MENU_BOX->labelsize(20);
    MAIN_MENU_BOX->labelcolor(WHITE);

    // first Menu Option Download Meter
    fltk::Button *mb1= new fltk::Button(10,40,200,30, " 1. Download ");//60
    mb1->labelfont(fltk::TIMES_BOLD);
    mb1->labelsize(16);
    mb1->argument(1);
    mb1->take_focus();
    mb1->color((fltk::Color)0x6E040600);
    mb1->align(fltk::ALIGN_CENTER);
    mb1->labelcolor(WHITE);
    mb1->box(ROUNDED_BOX);

  fltk::Input* input = new fltk::Input(400, 60, 120, 25, "key :");
  input->labelsize(14);
  input->callback((fltk::Callback*)cb_userinput, this);
  input->take_focus();
  input->argument(2);
  input->labelfont(HELVETICA_BOLD_ITALIC);
  input->textsize(14);
  input->textfont(fltk::COURIER_BOLD);
  input->when(fltk::WHEN_ENTER_KEY_CHANGED);


    // Second Menu Option to Print
    fltk::Button *mb2= new fltk::Button(10,80,200,30, " 0. Quit ");//60
    mb2->labelfont(fltk::TIMES_BOLD);
    mb2->labelsize(16);
    mb2->color((fltk::Color)0x6E040600);
    mb2->align(fltk::ALIGN_CENTER);
    mb2->labelcolor(WHITE);
    mb2->box(ROUNDED_BOX);



    Menu->end();
}
int MainMenu::GetUserOption()
{
    cout << endl << "Address of this:" << this << endl;
    getchar();  
    DisplayUserOptions();
     
    return user_choice;
}
MainMenu::~MainMenu()
{   
    delete Menu;
}

and my base class is

class IUserOptions
{
    protected:
        vector<string> UserChoices;
    public:
        IUserOptions(vector<string> userChoices);
        virtual int GetUserOption() = 0;
        virtual ~IUserOptions();
};