GUI: Beam intensity value validation is not working correctly

To reproduce:

1. Create a GISAS instrument
2. Set intensity to 1e33
3. Press tab to proceed to a different edit control

Actual behavior:
The value is not accepted, but this is not shown. 1e33 is still there. You have to switch to another instrument and back to this instrument to see that the actual value is not 1e33

Expected behavior:
The value is not accepted, but it has to be reflected in the edit control (show the real value)

Version of BornAgain
Is like this since 1.18.0