User is allowed to enter an invalid exercise to a training day

[chanalv]

Steps to Reproduce

1. When adding an exercise to a training day, leave the "Exercises" field blank or type in an invalid exercise that is not on the list of exercises.
2. Click "Save".

Expected results:
I expected the "Exercises" field to be required and to be prevented from submitting the form.

Actual results:
The form is submitted despite the blank or invalid exercise entry and creates an anomalous exercise under the training day, as seen in the image below.

Additional notes:
This issue can be reproduced in both the live system on https://wger.de/ and when running a development build (Git SHA-1: aaa822e). I tried implementing a quick fix by making the "Exercises" field required, as shown in the image below. It successfully prevents the user from submitting a blank or invalid exercise entry, but I couldn't get an error message to appear and it failed two of the tests, which makes me curious if there's a functional reason for the field to remain optional?

[AggelosK26]

Hey, can I be assigned this issue?