Show a warning if you try to grab focus when not allowed, closes #15388

scene/gui/control.cpp

@@ -2062,8 +2062,11 @@ void Control::grab_focus() {
 	if (!is_inside_tree()) {
 		ERR_FAIL_COND(!is_inside_tree());
 	}
-	if (data.focus_mode == FOCUS_NONE)
+
+	if (data.focus_mode == FOCUS_NONE) {
+		WARN_PRINT("This control can't grab focus. Use set_focus_mode() to allow a control to get focus.");
 		return;
+	}
 
 	get_viewport()->_gui_control_grab_focus(this);
 }