The upcoming version of DayPilot.Modal (2.8) will include a fix for this. You can also release the reference manually by calling "modal.iframe = null;" in onClosed event handler.
Note that if you reuse the DayPilot.Modal object it will be released automatically on the next showUrl() call.
Also, it looks like the DayPilot.Modal object remains in memory (you probably keep the reference somewhere). If you release the reference it will be cleared by the garbage collector including the internal iframe reference.