In Tapestry 18.104.22.168, you can have code such as:
private Form inputs;
This fails in 5.2.4 because getDefaultTracker() returns null if there are no errors. This appears to be related to this closed bug:
Perhaps getDefaultTracker() should check for a null and return an empty one to be backwards compatible with 22.214.171.124? Alternatively, document it as a known incompatibility in the release notes.