skip if not attached

Author: willmcgugan

src/textual/screen.py

@@ -1349,6 +1349,8 @@ def _on_screen_resume(self) -> None:
 
     def _on_screen_suspend(self) -> None:
         """Screen has suspended."""
+        if not self.is_attached:
+            return
         if self.app.SUSPENDED_SCREEN_CLASS:
             self.add_class(self.app.SUSPENDED_SCREEN_CLASS)
         self.app._set_mouse_over(None)