Can't dismiss new custom action sheet

OS X 10.11.3

Steps to reproduce:
1. Launch SourceTree;
2. Open Preferences;
3. Select "Custom Actions" tab;
4. Press "Add" button;
5. Press "Cancel" in newly presented sheet;

Expected Results:

• "New Custom Action" sheet is dismissed;

Actual Results:

• Sheet is still present

Even filling all the fields and then click "Ok" does not dismiss the sheet.