Fix spurious assert in wxMSW when destroying a TLW with buttons

This commit partially reverts 17055fb8c6
which resulted in asserts when destroying dialogs with buttons, one of
which had focus, see https://github.com/wxWidgets/wxWidgets/pull/1245

This happened because wxGetTopLevelParent() returns NULL when the parent
TLW was already half-destructed, resulting in an assert, while the
original code still returned NULL in this case, but did it silently,
which is the right thing to do here as we can't avoid being called
during the parent TLW destruction.

Closes https://github.com/wxWidgets/wxWidgets/pull/1268

src/msw/button.cpp

@@ -252,14 +252,26 @@ wxWindow *wxButton::SetDefault()
 // return NULL
 static wxTopLevelWindow *GetTLWParentIfNotBeingDeleted(wxWindow *win)
 {
-    wxWindow* const parent = wxGetTopLevelParent(win);
-    wxASSERT_MSG( parent, wxT("button without top level parent?") );
+    for ( ;; )
+    {
+        // IsTopLevel() will return false for a wxTLW being deleted, so we also
+        // need the parent test for this case
+        wxWindow * const parent = win->GetParent();
+        if ( !parent || win->IsTopLevel() )
+        {
+            if ( win->IsBeingDeleted() )
+                return NULL;
 
-    if ( parent->IsBeingDeleted() )
-        return NULL;
+            break;
+        }
+
+        win = parent;
+    }
+
+    wxASSERT_MSG( win, wxT("button without top level parent?") );
 
     // Note that this may still return null for a button inside wxPopupWindow.
-    return wxDynamicCast(parent, wxTopLevelWindow);
+    return wxDynamicCast(win, wxTopLevelWindow);
 }
 
 // set this button as being currently default