Prevent possible traceback

src/chat_control.py

@@ -1768,6 +1768,8 @@ class ChatControl(ChatControlBase):
 		if not c.resource:
 			return
 		if target_type == self.TARGET_TYPE_URI_LIST:
+			if not selection.data:
+				return
 			uri = selection.data.strip()
 			uri_splitted = uri.split() # we may have more than one file dropped
 			for uri in uri_splitted:

src/groupchat_control.py

@@ -1658,6 +1658,8 @@ class GroupchatControl(ChatControlBase):
 		# Invite contact to groupchat
 		treeview = gajim.interface.roster.tree
 		model = treeview.get_model()
+		if not selection.data:
+			return
 		data = selection.data
 		path = treeview.get_selection().get_selected_rows()[1][0]
 		iter = model.get_iter(path)