Make opening threads from a usercard opened with /usercard not crash the client (#3905)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/dialogs/UserInfoPopup.cpp

@@ -135,6 +135,8 @@ UserInfoPopup::UserInfoPopup(bool closeAutomatically, QWidget *parent,
     : DraggablePopup(closeAutomatically, parent)
     , split_(split)
 {
+    assert(split != nullptr &&
+           "split being nullptr causes lots of bugs down the road");
     this->setWindowTitle("Usercard");
     this->setStayInScreenRect(true);