wxWidgets (C++) Global Variable

[JustYakov]

I have two windows: one of them is the main window, and the other one is a special "Edit Names" dialog. The main window opens the file and the second dialog interprets it. The problem is that this code doesn't work because it constructs the window before initializing the variable:

Main window
void wxEVAFrame::OnLevelNames(wxCommandEvent& event)
{
LevelNames dlg(this);
dlg.Prep(filename); // <-filename is defined in the .h file
}
LevelNames:: Prep
void LevelNames::Prep(wxString fname){
filename = fname; // <-filename is defined in the .h file
wxMessageDialog(this,filename).ShowModal(); // This one displays the proper filename
ShowModal();
}
LevelNames::LevelNames also has the wxMessageDialog thing but it displays nothing.

Please help and/or explain why that is wrong.
Thanks in advance!