"Save configuration" crashes if a default button mapping has been cleared

[rbares]

The application should not crash when saving configuration.

Steps to reproduce:

• Open libCECTray
• Select the "Foreground application" tab
• Click the "Select" CEC Button
• Click "Clear" in the "Button configuration" dialog
• Click "Close" in the "Button configuration" dialog
• Select the "Configuration" tab
• Click "Save configuration"

Observed behaviour:

• Application crashes with a dialog warning that value cannot be null

Expected behaviour:

• Configuration is saved, without a mapping for Select