Find pattern widget: every second "find with regexp" task is not cancelled

Scenario to reproduce:

1. Open "data/samples/FASTA/human_T1.fa".
2. Open "Find pattern" options panel tab.
3. Set the "RegExp" algorithm and enter a pattern ".*".
   Expected state: a find task started.
4. Add several symbols 'A' to the pattern one by one.
   Expected state: after every symbol adding a new task starts, the previous one is canceled.
   Current state: not every previous task is canceled: only every second task.