fix some crashes and debug smtn

gtk-helpers.go

@@ -48,8 +48,14 @@ func switchToTab(jid string, w *gtk.Window) {
 
 	scroller.SetChild(typed_tab.msgs)
 	if typed_tab.isMuc {
-		m, _ := mucmembers.Load(jid)
-		ma := m.(mucUnit)
+		m, ok := mucmembers.Load(jid)
+		if !ok {
+			return
+		}
+		ma, ok := m.(mucUnit)
+		if !ok {
+			return
+		}
 		mm := ma.Members
 		gen := gtk.NewBox(gtk.OrientationVertical, 0)