diff --git a/gajim/common/app.py b/gajim/common/app.py
index bcccee4ec..33e29e96f 100644
--- a/gajim/common/app.py
+++ b/gajim/common/app.py
@@ -583,6 +583,9 @@ def prefers_app_menu():
 
 def get_app_window(cls):
     for win in app.get_windows():
-        if isinstance(win, cls):
+        if isinstance(cls, str):
+            if type(win).__name__ == cls:
+                return win
+        elif isinstance(win, cls):
             return win
     return None
diff --git a/gajim/disco.py b/gajim/disco.py
index 2076e0af5..9c7fb3062 100644
--- a/gajim/disco.py
+++ b/gajim/disco.py
@@ -1797,11 +1797,12 @@ class MucBrowser(AgentBrowser):
         if not iter_:
             return
         service = model[iter_][0]
-        if 'join_gc' not in app.interface.instances[self.account]:
+        window = app.get_app_window('JoinGroupchatWindow')
+        if window is None:
             app.interface.join_gc_minimal(self.account, service)
         else:
-            app.interface.instances[self.account]['join_gc'].set_room(service)
-            app.interface.instances[self.account]['join_gc'].present()
+            window.set_room(service)
+            window.present()
             self.window.destroy()
 
     def update_actions(self):