UI hang up in creating new tab/file

### Configuration

**IDE Version:** 1.6.12 (installed from ZIP file)
**OS:** Windows 10 Pro x64
**Language:** Czech
### Issue Description

When I try to create new tab/file (`CTRL+SHIFT+N`), tapping on OK and Cancel button doesn't work and input text field for entering file name hang up.
### Steps to reproduce
1. Create a new project
2. Add a new file/tab (`CTRL+SHIFT+N`)
3. Enter new file name
4. Press `Ok` or `Cancel`
5. Nothing happen and input text field hang up (keyboard keys `ENTER` and `ESC` works here OK)
### Workaround

Instead of `OK` / `Cancel` buttons use `ENTER` and `ESC` keyboard keys. When the UI hungs up, press `CTRL+SHIFT+N` to "un-hang up" the text field. :)

Start by reproducing the new-file dialog in Arduino IDE 1.6.12 on Windows 10 with the Czech language setting, following the listed keyboard and button steps. Trace the dialog's OK and Cancel handling and confirm that both buttons respond while Enter and Escape retain their current behavior.

Written by the indexing model from the issue text.