Primer Problem

[Stuart David]

Using Magenta Transformer colab notebook, I can't get it to accept my midi files for priming. 

It accepted my first one, a midi of Moonlight Sonata, but since then it just says "Upload widget is only available when this cell has been executed in the present browser session. Please run the cell to enable"

It still works if I upload moonlight sonata though.

What is the problem? This has happened over a series of days, with me setting up a new environment each time.