problem

At several places in the Javascript we use JSON.parse instead of Ext.JSON.decode.

If the parsed string is no valid JSON the error message of JSON.parse is
Uncaught SyntaxError: Unexpected token < in JSON at position 0

The error message of Ext.JSON.decode is instead:
[E] Ext.JSON.decode(): You're trying to decode an invalid JSON String: <test>

The latter is much more better for debugging.

In general: avoid usage of plain JS methods if the frame provide the same stuff!