ISSUEWhen a list, content type, site or site collection is deleted if there were Nintex Forms associated to any of the lists or content types etc they will still appear in the Nintex Mobile application.
RESOLUTIONFor Office 365 the local storage will need to be deleted from the settings in the Nintex Mobile Application and the forms re-downloaded.
For on-premise installations the first thing to try is to find the form in the NM_Form table in the Nintex Forms database and change the IsDeleted column from 0 to 1.
If the form still appears in the Nintex Mobile app the only other option is to delete the whole row in the Nintex Forms Database for the form.
ADDITIONAL INFORMATIONThe reason that this issue occurs is because Nintex is a third party application and SharePoint does not tell Nintex when one of its forms is deleted.
To avoid this issue all together Nintex Forms should be deleted by opening them in the Nintex Forms designer and then clicking delete form before the SharePoint asset is deleted.