Yes, you are right. Now I modified the code - at first "Oninitialize", then "Show". But it displays only the "dcl_messagebox" which is in the "defun", and nothing else happens. Also the commandline brings "nil")
Befehl: (dcl_Grid_SetCellText grid_prob_dialog_GridUeber 2 1 "Opendcl")
nil
EDIT: Now I found why I get the error with the "argument" - I started the code above while the dialogue was not(!) displayed, and this makes the error.
If it is displayed I get "nil"..