fix: hot reloading broke because we did not restore the previous context

solara/server/app.py

@@ -31,6 +31,13 @@
 reload.reloader.start()
 
 
+class Local(threading.local):
+    app_context_stack: Optional[List[Optional["AppContext"]]] = None
+
+
+local = Local()
+
+
 class AppType(str, Enum):
     SCRIPT = "script"
     NOTEBOOK = "notebook"
@@ -270,12 +277,16 @@ def display(self, *args):
         print(args)  # noqa
 
     def __enter__(self):
+        if local.app_context_stack is None:
+            local.app_context_stack = []
         key = get_current_thread_key()
+        local.app_context_stack.append(current_context.get(key, None))
         current_context[key] = self
 
     def __exit__(self, *args):
         key = get_current_thread_key()
-        current_context[key] = None
+        assert local.app_context_stack is not None
+        current_context[key] = local.app_context_stack.pop()
 
     def close(self):
         with self: