Check document size after selecting document from uploader in a form sail

I have a form sail with a fileUploadField. When I select a file, I need to check the size and disable a button if is greather than 1MB.
The check is the following:

When a perform this check an error appear:

Interface Definition: Expression evaluation error: An error occurred while executing a save: Expression evaluation error at function 'document' [line 886]: Error evaluating function 'document' : Document Does Not Exist or has been Deleted

How can I resolve this problem?


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