Pattern enter field is still red even after the entered regexp becomes valid

Scenario to reproduce:

1. Open "_common_data/fasta/fa1.fa".
2. Open "Search in sequence" options panel tab. Select "RegExp" algorithm.
3. Enter next regexp:

   (
   

   Expected state: the pattern enter field becomes red.

4. Remove entered pattern, enter a valid pattern:

   .
   

   Expected state: the pattern enter field is not colored.
   Current state: the pattern enter field is still red.