Fix crash changing clip monitor bg color when no clip is selected

src/monitor/monitor.cpp

@@ -691,7 +691,11 @@ void Monitor::slotForceSize(QAction *a)
     updateGeometry();
 }
 
-void Monitor::buildBackgroundedProducer(int pos) {
+void Monitor::buildBackgroundedProducer(int pos)
+{
+    if (m_controller == nullptr) {
+        return;
+    }
     if (KdenliveSettings::monitor_background() != "black") {
         Mlt::Tractor trac(pCore->getCurrentProfile()->profile());
         QString color = QString("color:%1").arg(KdenliveSettings::monitor_background());