prevent a case where a stepTimer listener could get removed twice as a side effect of voicingManager.cancel(), see https://github.com/phetsims/john-travoltage/issues/445
prevent a case where a stepTimer listener could get removed twice as a side effect of voicingManager.cancel(), see https://github.com/phetsims/john-travoltage/issues/445

phetsims/scenery,phetsims/scenery,phetsims/scenery