Hii Shkelqim,
You are getting dump not because of your MANDT field addition but because of object grid.As it's illustrated in the dump , you are accessing the method SET_TABLE_FOR_FIRST_DISPLAY of class CL_GUI_ALV_GRID via grid1 which is initial means not created or not assigned.
Make sure you have not commented out the object grid1 creation.
While creating object grid1 of cl_gui_alv_grid to access the methods of cl_gui_alv_grid pass i_parent parameter value as container name in which you are displaying the grid.
Regards
Syed