In Tapestry 126.96.36.199, 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 188.8.131.52? Alternatively, document it as a known incompatibility in the release notes.