Missing `loc` due to deprecation of `_debugSource`

[jamieo-ms]

In newer versions of React, the _debugSource property was removed from fiber nodes: https://github.com/facebook/react/blob/58ac15cdc953a21ac1b352b6300abc3da87fa3d6/packages/react-devtools-shared/src/utils.js#L370

This causes all loc information to be null in the mount events.