Fix crash happening when QuickSwitcher is used with popout window (#4187)

Co-authored-by: Felanbird <41973452+Felanbird@users.noreply.github.com>
Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

src/widgets/Notebook.hpp

@@ -90,7 +90,7 @@ private:
     void resizeAddButton();
 
     bool containsPage(QWidget *page);
-    Item &findItem(QWidget *page);
+    Item *findItem(QWidget *page);
 
     static bool containsChild(const QObject *obj, const QObject *child);
     NotebookTab *getTabFromPage(QWidget *page);