It sounds like the While loop is using one of these "deleted" elements for its verification. Once they are all gone, that element no longer exists, so Test Studio cannot find it to make its comparison and properly exit the loop. Thus you receive the "Unable to find element" error.
Perhaps there is a different element to base the While loop's verification on. Is there a field that displays the number of currently visible items? If so, like the demo video, you could craft the verification to 'InnerText' 'NotContain' '0'. So, while that count number is not 0, the loop continues executing (deleting the items) until the count is 0, and then it exits.
Without seeing your app first-hand I can only speculate how to alter your test to make it run properly. If you continue to have difficulty, please grant me remote access and attach a copy of your test in .zip format.
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