Merge branch 'release/21.04' addendum

This should have belonged to my previous merge
480b3067 but somehow I managed to
miss this one line.

It was written by Derek Christ as part of
dc3beae3.

src/dolphincontextmenu.cpp

@@ -64,7 +64,7 @@ DolphinContextMenu::DolphinContextMenu(DolphinMainWindow* parent,
     const DolphinView* view = m_mainWindow->activeViewContainer()->view();
     m_selectedItems = view->selectedItems();
 
-    installEventFilter(this);
+    QApplication::instance()->installEventFilter(this);
 
     static_cast<KHamburgerMenu *>(m_mainWindow->actionCollection()->
                 action(QStringLiteral("hamburger_menu")))->addToMenu(this);