[libtaskmanager] Fix null check for window

window() may be null. We check for that, but 5a3ef9e825 introduced code that uses window() before the check.

Move the check before the relevant code to fix that

BUG: 456094

BUG: 457690

libtaskmanager/declarative/pipewiresourceitem.cpp

@@ -231,12 +231,18 @@ void PipeWireSourceItem::updateTextureDmaBuf(const QVector<DmaBufPlane> &planes,
         qCWarning(PIPEWIRE_LOGGING) << "glEGLImageTargetTexture2DOES is not available" << window();
         return;
     }
+
+    if (!window()) {
+        qCWarning(PIPEWIRE_LOGGING) << "Window not available" << this;
+        return;
+    }
+
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     const auto openglContext = window()->openglContext();
 #else
     const auto openglContext = static_cast<QOpenGLContext *>(window()->rendererInterface()->getResource(window(), QSGRendererInterface::OpenGLContextResource));
 #endif
-    if (!window() || !openglContext || !m_stream) {
+    if (!openglContext || !m_stream) {
         qCWarning(PIPEWIRE_LOGGING) << "need a window and a context" << window();
         return;
     }