When CRUD is submitted, the following PHP notice is shown:

<b>Notice</b>:  Undefined offset: 0 in <b>.../vendor/atk4/ui/src/jsCallback.php</b> on line <b>144</b><br />

It seems that:
a) the $response is an array with one value but instead of with index 0, the index is 1
b) adding extra test if the index 0 exists seems to fix this issue

Questions:
a) What does the 0 index mean?
b) what is the purpose of the extra code when index 0 is presented?

How to fix this?