Docker: Add sanity check in start container

Change-Id: I633685402f27a3af317a58fd6f3c1d58174db989
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/docker/dockerdevice.cpp

@@ -753,6 +753,8 @@ bool DockerDevicePrivate::startContainer()
                                              "or restart Qt Creator."));
     });
 
+    QTC_ASSERT(m_shell, return false);
+
     if (m_shell->start())
         return true;