screencast: Disable screencasting when a window closes

There were still some cases where we might have queried the window,
leading to a crash.

BUG: 469055

src/plugins/screencast/screencaststream.cpp

@@ -313,7 +313,10 @@ ScreenCastStream::ScreenCastStream(ScreenCastSource *source, QObject *parent)
     , m_source(source)
     , m_resolution(source->textureSize())
 {
-    connect(source, &ScreenCastSource::closed, this, &ScreenCastStream::stopStreaming);
+    connect(source, &ScreenCastSource::closed, this, [this] {
+        m_streaming = false;
+        Q_EMIT stopStreaming();
+    });
 
     pwStreamEvents.version = PW_VERSION_STREAM_EVENTS;
     pwStreamEvents.add_buffer = &ScreenCastStream::onStreamAddBuffer;