Endless loop in EditTableDialog::addField() #941

Details for the issue

1. Create new Table;
2. Click 'Add field' in 'Edit table definition' dialog;
3. DB4S isn't responding.

Useful extra information

Bug was introduced by @MKleusberg in f728ff7

I'm opening this issue because:

• DB4S is hang up
• DB4S has a bug
• DB4S needs a feature
• DB4S has another problem

I'm using DB4S on:

• Windows: ( version: ___ )
• Linux: ( distro: ___ )
• Mac OS: ( version: ___ )
• Other: ___

I'm using DB4S version:

• 3.9.1
• 3.9.0
• Other: ___

I have also:

• Tried out the nightly version: https://github.com/sqlitebrowser/sqlitebrowser#windows
• Searched for an existing similar issue: https://github.com/sqlitebrowser/sqlitebrowser/issues?utf8=%E2%9C%93&q=is%3Aissue%20