Throws error on burst image capture when user inputs bad expression index

[japamat0]

Description:
Type Error thrown when trying capture burst of images on certain graphs, reads 'cannot read property 'match' of undefined.

the GraphingCalculator.getExpressions() method from the API returns an array of expressions on the current state of the calculator, though not every object element of the array returned has a property latex, compare 0 & 1 below:

For context

Possible Fix
Add a conditional statement to ensure that the retrieved expression has the latex property, otherwise return 'null'

Reproduction Steps:

1. Go to gifsmos.com
2. Paste in the following URL https://www.desmos.com/calculator/cbt18evgoe
3. Click Burst icon to open the burst capture panel & click capture
4. Error will appear in console, screenshot below

[japamat0]

UPDATE:
setSliderByIndex() for valid index should also check expression object is of type expression before trying match