TelemetryPacket.addData() returns null instead of an Item

[cmacfarl]

https://github.com/acmerobotics/ftc-dashboard/blob/master/FtcDashboard/src/main/java/com/acmerobotics/dashboard/telemetry/TelemetryPacket.java#L107

This results on a followon call to MultipleTelemetry.setValue() throwing a NullPointException when it iterates unchecked through the items list which contains 2 items, one of which is null.