Exit GUI Plugin

Sorry if this is a stupid question: But how do I exit a GUI plugin without starting the main dialog if an error occurs during initialization?

Example: If the user has not selected a book, I print the following message in the __init__ method of class MyGuiPluginDialog(QDialog):

This will display the Calibre error dialog. So good so far...

But how do I stop executing the plugin at this point?

"return" shows the error dialog again, after "OK" the main dialog of the plugin is shown.
"sys.exit()" closes Calibre itself.