Fix javascript error with custom tabs and page providers

NXP-28161 has introduced this regression

1. setup a pageprovider with an aggregate on the document type
2. make the search form and results appear in a tab
3. navigate to the newly created tab

Observed behavior:

• the tab content is not correctly displayed
• a javascript error is logged in the console
• the filter count is negative

Expected behavior:

• the tab is correctly displayed
• the filter count is correct (and never negative)