Unhandled exception in Filter Events interface

[rbauststfc]

Describe the bug

It's possible to trigger an unhandled exception when entering start and stop times in the Filter Events interface.

To Reproduce

1. Go to Interfaces -> Utility -> Filter Events.
2. Delete the entry in the Stopping Time field (under the plot). Leave the entry in the Starting Time field.
3. Put your cursor into the Starting Time field and hit enter on the keyboard.

It also works the other way around (i.e. clear Starting Time and hit enter in Stopping Time).

Expected behavior

There should be no exception.

Platform/Version (please complete the following information):

• Mantid Version: introduced in version 6.10. The interface seemed to change a bit in this version.

[rbauststfc]

@sf1919 we found this during manual testing. It was introduced in 6.10 and I don't think is urgent to fix for 6.12. I'm flagging it because it will need a decision about who should pick it up and when, though.