Pressing the ESCAPE key in the "check mode" causes the application to terminate with an exception

Steps to reproduce

Run the application with the following command:
```
gramma check "."
```
Press the ESCAPE key.

Actual behavior

The application terminates, and the output contains a stack trace, starting with the following:

(node:12393) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'value' of undefined
...

Expected behavior

I do not know whether all the described behavior or only part of it is a bug. The README does not contain information about possible behavior of any key. Possibilities of expected behavior are:

If all of the behavior is a bug – the ESCAPE key should not cause the application to terminate – then the expected behavior is both not to terminate, and to print some appropriate message (e.g. This key is not handled) or to print nothing at all.
If only part of the behavior is a bug – the ESCAPE key should terminate the application, but there should be no stack trace printed – then the expected behavior is just to print some appropriate message (e.g. This key is not handled) or to print nothing at all.

Notes

As I have observed, this behavior happen not only using gramma check, but I have not checked all the possible cases, so I leave for the author finding the scope of this behavior.

UPDATE: This problem happens also with the combination CTRL+D. But, since I cannot currently make the application work, I cannot confirm whether the stack trace is the same. I will try that later.

caderek / gramma