Fix issue with group.setlayout and TreeTab

When using `group.setlayout` to set the `TreeTab` layout, the side panel
is not rendered at all.

We can fix this by using the existing method for switching layouts,
rather than using the existing approach to set the layout.

Fixes qtile#5029

libqtile/group.py

@@ -123,9 +123,7 @@ def layout(self, layout):
         """
         for index, obj in enumerate(self.layouts):
             if obj.name == layout:
-                self.current_layout = index
-                hook.fire("layout_change", self.layouts[self.current_layout], self)
-                self.layout_all()
+                self.use_layout(index)
                 return
         logger.error("No such layout: %s", layout)
 
@@ -135,8 +133,9 @@ def use_layout(self, index: int):
         self.current_layout = index % len(self.layouts)
         hook.fire("layout_change", self.layouts[self.current_layout], self)
         self.layout_all()
-        screen_rect = self.screen.get_rect()
-        self.layout.show(screen_rect)
+        if self.screen is not None:
+            screen_rect = self.screen.get_rect()
+            self.layout.show(screen_rect)
 
     def use_next_layout(self):
         self.use_layout((self.current_layout + 1) % (len(self.layouts)))