Custom expression editor: fix Enter not committing the change

[ariya]

This fixes issue #15241. The solution is to ensure that the state change is synchronized with the commit event propagation.

To verify:

1. Ask a question, Custom question
2. Sample Dataset, Products table
3. Add filters to narrow your search
4. Choose Price, Greater than 1, click Add Filter
5. Click on the new "Price is greater than 1" filter
6. Click on the left chevron arrow, Custom Expression
7. Complete the expression to [Price] > 1 AND [Price] < 5
8. Press Enter

Before:

After:

[daltojohnso]

Very nice