diff --git a/lcl/interfaces/gtk2/gtk2winapi.inc b/lcl/interfaces/gtk2/gtk2winapi.inc
index a5c247c210..28c7d08b4f 100644
--- a/lcl/interfaces/gtk2/gtk2winapi.inc
+++ b/lcl/interfaces/gtk2/gtk2winapi.inc
@@ -6536,7 +6536,7 @@ begin
   if GtkWindow = nil then
     Exit;
 
-  Result := (PGtkWidget(GtkWindow)^.Window<>nil)
+  Result := (PGtkWidget(GtkWindow)^.Window<>nil) and GDK_IS_WINDOW(PGtkWidget(GtkWindow)^.Window)
       and (gdk_window_get_state(PGtkWidget(GtkWindow)^.Window)
            and GDK_WINDOW_STATE_ICONIFIED <> 0);
 end;
@@ -6600,7 +6600,7 @@ begin
   if GtkWindow = nil then
     Exit;
 
-  Result := gdk_window_get_state(PGtkWidget(GtkWindow)^.Window) and GDK_WINDOW_STATE_MAXIMIZED <> 0;
+  Result := GDK_IS_WINDOW(PGtkWidget(GtkWindow)^.Window) and (gdk_window_get_state(PGtkWidget(GtkWindow)^.Window) and GDK_WINDOW_STATE_MAXIMIZED <> 0);
 end;
 
 {------------------------------------------------------------------------------
diff --git a/lcl/interfaces/gtk2/gtk2wsforms.pp b/lcl/interfaces/gtk2/gtk2wsforms.pp
index 9a2e0b95f4..54f888409c 100644
--- a/lcl/interfaces/gtk2/gtk2wsforms.pp
+++ b/lcl/interfaces/gtk2/gtk2wsforms.pp
@@ -794,7 +794,7 @@ begin
       (AForm.PopupParent = nil) then
     begin
       TempGdkWindow := {%H-}PGdkWindow(Gtk2WidgetSet.GetForegroundWindow);
-      if (TempGdkWindow <> nil) and (GdkWindowObject_modal_hint(GDK_WINDOW_OBJECT(TempGdkWindow)^) = 0) then
+      if (TempGdkWindow <> nil) and GDK_IS_WINDOW(TempGdkWindow) and (GdkWindowObject_modal_hint(GDK_WINDOW_OBJECT(TempGdkWindow)^) = 0) then
       begin
         if ((gdk_window_get_state(TempGdkWindow) and GDK_WINDOW_STATE_ABOVE) = GDK_WINDOW_STATE_ABOVE) or
           GTK2WidgetSet.GetAlwaysOnTopX11(TempGdkWindow) then