Fix previous sink plugin GUI delete by doing it explicitly in the main window

sdrgui/mainwindow.cpp

@@ -300,6 +300,11 @@ void MainWindow::addSinkDevice()
 
     // create a file sink by default
     m_pluginManager->selectSampleSinkBySerialOrSequence("sdrangel.samplesink.filesink", "0", 0, m_deviceUIs.back()->m_deviceSinkAPI);
+
+    // delete previous plugin GUI if it exists
+    m_deviceUIs.back()->m_deviceSinkAPI->getPluginInterface()->deleteSampleSourcePluginInstanceGUI(
+            m_deviceUIs.back()->m_deviceSinkAPI->getSampleSinkPluginInstanceGUI());
+
     DeviceSampleSink *sink = m_deviceUIs.back()->m_deviceSinkAPI->getPluginInterface()->createSampleSinkPluginInstanceOutput(
             m_deviceUIs.back()->m_deviceSinkAPI->getSampleSinkId(), m_deviceUIs.back()->m_deviceSinkAPI);
     m_deviceUIs.back()->m_deviceSinkAPI->setSampleSink(sink);

sdrgui/plugin/pluginmanager.cpp

@@ -498,7 +498,6 @@ int PluginManager::selectSampleSinkBySerialOrSequence(const QString& sinkId, con
             << " seq: " << m_sampleSinkDevices[index].m_deviceSequence;
 
     deviceAPI->stopGeneration();
-    deviceAPI->setSampleSinkPluginInstanceUI(0); // this effectively destroys the previous GUI if it exists
 
 	//	m_sampleSourcePluginGUI = pluginGUI;
     deviceAPI->setSampleSinkSequence(m_sampleSinkDevices[index].m_deviceSequence);