1. Fix bug in EvaluatorDialog and arrows in code completion.

2. Fix exception when deleting text in EvaluatorDialog

How to reproduce for fix 2: 
  1. Write object name
  2. Hit '.' - code completion widget is shown
  3. Hit Backspace few times exception is thrown.

How to reproduce for fix 1;
  1. Write object name
  2. Hit '.' - code completion widget is shown
  3. Select object property using arrows and hit enter
  4. Delete selected property name up to the point
  5. Try to use arrows in completion widget. They don't work.