Equation becomes blank and yellow after getting a yellow equation followed by deleting node

[jonwetzel]

Steps:

1. Open rabbits in student mode
2. Create the accumulator ("population") and get the equation wrong twice. (E.g. "death rate" -> "- death rate") You should get a yellow net growth as expected.
3. Click done, then delete all nodes except the accumulator. When you delete "Net Growth", check the console--4 or 5 Assertion Failed errors appear related to that node id. (maybe this causes the bug?)
4. Now delete the accumulator.
5. Create the accumulator for "population" again. Enter "net growth" and click Check expression.
   The equation gets filled in with "net growth" and turns yellow, but the net growth node is not created. (The effect of the BUG)
6. Re-open the accumulator to see the blank equation in yellow state. The user is now stuck.

[jonwetzel]

Find steps to reproduce this bug by checking user sessions, steps table.

[vanlehn]

Adding semantic check on close and check expresssion

[jonwetzel]

Follow up: Dragoon-Lab/topomath#415

[jonwetzel]

I've discovered a set of steps to reliably reproduce this bug. Adding them to the top comment....