Wild guess: t is supposed to be the hook argument with the statement ID, which doesn’t exist yet for newly added statements? That might be why so far I haven’t seen this error when editing or removing existing statements.

Yeah, I think that’s more or less it. Hook arguments for a new statement (on test wikidata, logged with mw.hook( 'wikibase.statement.saved' ).add( console.log )):

Q117701
Q117701$d18c20ce-4a85-13a1-7ce8-0ba5fd15c69b
null
Object { _claim: {…}, _references: {…}, _rank: 1 }

Tainted References’ MWHookHandler:

	private addSaveHook( store: Store<Application> ): void {
		this.mwHooks( 'wikibase.statement.saved' ).add(
			( _entityId: string, statementId: string, oldStatement: Statement, newStatement: Statement ) => {
				if ( store.state.statementsTaintedState[ statementId ] ) {
					store.dispatch( STATEMENT_TAINTED_STATE_UNTAINT, statementId );
				} else if ( this.taintedChecker.check( oldStatement, newStatement ) ) {
					store.dispatch( STATEMENT_TAINTED_STATE_TAINT, statementId );
				}
			},
		);
		this.mwHooks( 'wikibase.statement.saved' ).add(
			( _entityId: string, _statementId: string, oldStatement: Statement, newStatement: Statement ) => {
				this.statementTracker.trackChanges( oldStatement, newStatement );
			},
		);
	}

The TaintedChecker:

	public check( oldStatement: Statement, newStatement: Statement ): boolean {
		return !( oldStatement.getClaim().getMainSnak().equals( newStatement.getClaim().getMainSnak() ) ) &&
			oldStatement.getReferences().equals( newStatement.getReferences() ) &&
			!oldStatement.getReferences().isEmpty();
	}

The StatementTracker:

	public trackChanges( oldStatement: Statement, newStatement: Statement ): void {
		const referenceChangeCount = this.refChangeCounter.countOldReferencesRemovedOrChanged(
			oldStatement.getReferences(),
			newStatement.getReferences(),
		);
		const oldRefCount = oldStatement.getReferences().length;
		const newRefCount = newStatement.getReferences().length;
		// ...
	}

Nothing here is prepared for the oldStatement being null.

Random-ish example of a broken user script (found via search): @matej_suchanek’s moveClaim.js is supposed to add the “move claim” button to newly added statements, but currently doesn’t. (The rest of the gadget probably still works, I haven’t tested it.)

Does this only happen in the tainted reference situation (i.e. changing statement value without changing the reference), or on every statement edit?

It happens only not in the tainted reference situation, because if you’re changing the statement value (with or without changing the reference), then oldStatement won’t be null.

However, due to T250069, once this has happened anywhere on the page (i. e. you’ve added any new statement), the hook will never reach subscribers again until the page is reloaded, for tainted reference-eligible edits as well as other statement edits.

Change 591376 had a related patch set uploaded (by Lucas Werkmeister (WMDE); owner: Lucas Werkmeister (WMDE)):
[mediawiki/extensions/Wikibase@master] TR: Handle oldStatement being null in hook handler

https://gerrit.wikimedia.org/r/591376

Change 591376 merged by jenkins-bot:
[mediawiki/extensions/Wikibase@master] TR: Handle oldStatement being null in hook handler

https://gerrit.wikimedia.org/r/591376