fix GroupchatControl session-related TB

src/chat_control.py

@@ -1119,7 +1119,6 @@ class ChatControl(ChatControlBase):
 
 		if not session:
 			session = gajim.connections[self.account].find_controlless_session(self.contact.jid)
-			self.session = session
 
 		if session:
 			session.control = self

src/message_control.py

@@ -43,6 +43,8 @@ class MessageControl:
 		self.hide_chat_buttons = False
 		self.resource = resource
 
+		self.session = None
+
 		gajim.last_message_time[self.account][self.get_full_jid()] = 0
 
 		self.xml = gtkgui_helpers.get_glade('message_window.glade', widget_name)