The page disappeared in frontend editor

After waiting 4 days and working for several hours, I think I have reached the problem - and a temporary solution, thank God -

The problem is that one of the articles that appears on the page has an invalid UTF character, and since the code for showing the page in the frontend editor depends on json_encode function, this led to an error and returned the value FALSE from the json_encode function, which led to the disappearance of the page from the frontend editor completely.

the solution:

1- Delete the invalid UTF character - or characters - from the article. But this requires that you know what article contains these invalid characters

or

2- A temporary solution so that the support team can solve the problem fundamentally:
I opened the file /administrator/components/com_sppagebuilder/builder/classes/addon.php
And changed all
json_encode( $datas )
to
json_encode( $datas, JSON_INVALID_UTF8_IGNORE )
There are 3 occurrences in the file