The error message you see (Full HTML: Missing filter. All text is removed.) means that one of the filters you selected on admin/config/content/formats/manage/full_html is missing; this usually happens when you uninstall the module implementing it. As explained in Missing filters result in Exception when the format is used, in those cases, Drupal doesn't show the text using the input format containing the missing filter.
That is not a problem. Just go to admin/config/content/formats/manage/full_html and save; the reference to the missing filter will be removed, and you will be able to see the body text again. In fact, the form builder for the input format form contains the following code. (See FilterFormatFormBase::form().)
// Create filter plugin instances for all available filters, including both
// enabled/configured ones as well as new and not yet unconfigured ones.
$filters = $format->filters();
foreach ($filters as $filter_id => $filter) {
// When a filter is missing, it is replaced by the null filter. Remove it
// here, so that saving the form will remove the missing filter.
if ($filter instanceof FilterNull) {
drupal_set_message($this->t('The %filter filter is missing, and will be removed once this format is saved.', array('%filter' => $filter_id)), 'warning');
$filters->removeInstanceID($filter_id);
}
}
The message clearly says The filter is missing, and will be removed once this format is saved. So, saving should fix the issue.
If this doesn't work, remove another filter from the input format, save, re-add the same filter you removed, and save again. At least, this should clear the eventual cache Drupal uses for filters or input formats.
