This error is displayed when a widget or operator raises an exception while processing an event (in your case, an event that came from the "input" input endpoint).
You should be able to obtain more details about the exception clicking on the "Details" section that appears near the log entry:

In this case, the exception was raised at line 15 of the js/main.js file. Take into account that currently WireCloud doesn't remove the part of the stack trace related to the internal code of WireCloud (the stack trace part of WireCloud starts at the line of Endpoint.propagate), we are working on removing it to make more clear those stack traces.
You can also debug those problems by using the developers tools provided by your browser. You only need to ensure your developer tools are configured to stop on any exception (including caught ones). E.g. on Chromium/Chrome:
