Libarycontrol: backport of #3198: fix crash when trying to refocus th…

…e library while another Mixxx window has focus

src/library/librarycontrol.cpp

@@ -405,16 +405,25 @@ void LibraryControl::emitKeyEvent(QKeyEvent&& event) {
 }
 
 void LibraryControl::setLibraryFocus() {
-    if (m_pSidebarWidget) {
-        // XXX: Set the focus of the library panel directly instead of sending tab from sidebar
-        m_pSidebarWidget->setFocus();
+    // XXX: Set the focus of the library panel directly instead of sending tab from sidebar
+    VERIFY_OR_DEBUG_ASSERT(m_pSidebarWidget) {
+        return;
+    }
+    // Try to focus the sidebar.
+    m_pSidebarWidget->setFocus();
+    // This may have failed, for example when a Cover window has focus,
+    // so make sure the sidebar is focused or we'll crash.
+    if (m_pSidebarWidget->hasFocus()) {
+        // Send Tab to move focus to the Tracks table.
+        // Obviously only works as desired if the skin widgets are arranged
+        // accordingly.
         QKeyEvent event(QEvent::KeyPress, Qt::Key_Tab, Qt::NoModifier);
         QApplication::sendEvent(m_pSidebarWidget, &event);
     }
 }
 
 void LibraryControl::slotSelectSidebarItem(double v) {
-    if (m_pSidebarWidget == NULL) {
+    VERIFY_OR_DEBUG_ASSERT(m_pSidebarWidget) {
         return;
     }
     if (v > 0) {