error handling: wait for error-text (%editor) to unlock if locked.

error dialog may be [locked](http://docs.racket-lang.org/gui/editor-overview.html#%28part._lockinfo%29) when exception occurs, this may result in following errors:
```
sequence-contract-violation: negative: 
method set-max-width cannot be called except in states (unlocked write-lock)
```
make explicit wait for it to unlock editor before sending text to it.