Fix forwarding of menu commands in the richtext sample

Fall back on the main rich text control if there is no currently focused
window, as the control itself never has focus when a menu is opened with
wxGTK/Wayland, i.e. none of the menu commands, such as wxID_COPY/CUT,
worked for it in this case.

Closes #22121.

samples/richtext/richtext.cpp

@@ -757,6 +757,8 @@ MyFrame::MyFrame(const wxString& title, wxWindowID id, const wxPoint& pos,
         const wxSize& size, long style)
        : wxFrame(NULL, id, title, pos, size, style)
 {
+    m_richTextCtrl = NULL;
+
 #ifdef __WXMAC__
     SetWindowVariant(wxWINDOW_VARIANT_SMALL);
 #endif
@@ -1339,6 +1341,8 @@ bool MyFrame::ProcessEvent(wxEvent& event)
             s_id = event.GetId();
 
             wxWindow* focusWin = wxFindFocusDescendant(this);
+            if (!focusWin)
+                focusWin = m_richTextCtrl;
             if (focusWin && focusWin->GetEventHandler()->ProcessEvent(event))
             {
                 //s_command = NULL;