DeviceSourceAPI: effectively delete the UI when changing the PluginInstanceGUI reference

sdrgui/device/devicesourceapi.cpp

@@ -176,6 +176,10 @@ void DeviceSourceAPI::setSampleSourcePluginInterface(PluginInterface *iface)
 
 void DeviceSourceAPI::setSampleSourcePluginInstanceGUI(PluginInstanceGUI *gui)
 {
+    if (m_sampleSourcePluginInstanceUI && (gui != m_sampleSourcePluginInstanceUI)) {
+        m_sampleSourcePluginInstanceUI->destroy();
+    }
+
     m_sampleSourcePluginInstanceUI = gui;
 }