Quicktime error

I deleted all the files under that name

This is the first time I've heard of this particular problem since no one has previously provided the follow-up info you posted. This likely explains the problem. I am only guessing here, but it sounds as if you deleted file(s) referenced in the QTX savedState PLIST file before clearing the already stored reference(s) and before telling System Preferences not to "keep" this information. As a result, reference(s) pointing to the missing file(s) you deleted is/are "orphaned"—causing the QTX Player app to crash upon any attempt to open and reload the no longer existent data. If we assume your system is configured/storing the PLIST file in the same location as my system, then try deleting the following file before opening and then closing the QTX Player app to generate a new PLIST file devoid of such references:

"~/Users/~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Saved Application State/com.apple.QuickTimePlayerX.savedState/windows.plist"

If you are not familiar with how to get to your "invisible" user account library file, simply hold down the 'Option' key while selecting the 'Library' option in the Finder "Go" sub-menu. Then navigate to the specified PLIST file, select it, and delete it as depicted in my "QuickieTutorial" here: QuickTip-DeleteSavedState.mp4

Every time I try to use quick player it opens, converting and then quits.

What do I do?

If you can, abort/close the QTX app with the 'Option' key held down before it can quit on its own to prevent the QTX app from attempting to re-open the file it is now trying to convert the next time the app is opened. Alternatively, you can turn on the global "System Preferences... > General > 'Close windows when quitting an app'" option as depicted below:

It quits too quickly. And that box is already ticked

What happens if you move/hide the file in an inaccessible path location? If it stops and gives you a modal message saying it can't find the file are you then able to acknowledge the message and close the app, forcing it to "forget" the currently open player?

I don't know what the file is. I am just trying to open quick player from the application list

According to your own screen capture image the filename begins with a "bUnknown-5c0209ed-7c9b-4317-a345-25de68" series of characters. Surely you don't have that many files of your system using this partial filename. (I.e., try a "Spotlight" search on the partial filename.)