It's not nice to have such errors:

Steps to reproduce:
1. Open a modal window form (for example hosts form).
2. Press "Update" and quickly press "Cancel".
3. See JS error in console.

When a request is sent, we cannot stop the execution. Best we could do is deal with the response.

There are some things we could do:
a) Disable the "Cancel" button, since it doesn't really cancel the process anyway. It just closes the window. Same for "X" on the top right corner. Wait for response and then close the window on success or show error in failure just like before. If tired of waiting press F5 to reload the page. It should close the window regardless of what happened.
b) Capture the response in main page, if there is no modal window available. It's similar to how success message is displayed after an object was created or updated. Perhaps the best choice.
c) Cancel receiving the response from request and not show anything. Reason here is that if we pressed "Cancel", it says that we no longer care about what's going on whether the request succeeded or failed.