Error with locking and unlocking variables - evaluating variable: Cannot read properties of null (reading 'tok')

[Ben Brawn]

There is a bug with locking and unlocking variables in the editor.

Here are the steps to reproduce the issue (don't do it on a question you like!) 

1. click the padlock to lock a variable
2. unclick the padlock to unlock a variable

Note

1. now in the editor the table has nothing in 'type' and 'generated value'
2. interestingly the question will still run (including any randomiation of that variable)
3. deleting the variable and reusing the same name doesn't solve the issue 
4. if you make a new variable, say blah, that depends on the now dodgy variable you will get the error "Error evaluating variable blah: Cannot read properties of null (reading 'tok')"

Here is an example

https://numbas.mathcentre.ac.uk/question/share/edit/e44ca5da-0113-41ec-a16d-cf9b8a0aee6b

[Ben Brawn]

Unexpectedly (for me), if you close the question and reopen it, then it works again.

[Ben Brawn]

Oh and I missed two steps in producing the bug, looks like you need to click "regenerate values" while the padlock is on and then click "regenerate values" once you turn the padlock off.

[Christian Lawson-Perfect]

Thanks for spotting this! I think that this must have been due to a change in how the variable scope used to evaluate variables is created.

I've fixed the problem.

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