decoding extrinsic panics.

[georlav]

Every time I try to use codec.Decode or codec.DecodeFromHex on an extrinsic I previously created using the codec, the codec panics. I can broadcast these extrinsics without any issues, so they are valid.

In the following example, I'm using codec.DecodeFromHex for simplicity, but the same problem occurs if I use codec.Decode or even if I try to use the Marshaler.

encodedExt  = "0x450284005cd06e12b185738b6c25feb5ce37d522ef887faed480ccd4bf970fff094b3a5c01e4cdece2479208010bd5dbfc0851261a16bc9c7d3e63d69a3d148f31bd9a3f6ed0ae5d36ea1905dfcb64e98f8ff09e5576da86428ab27947a9c27ab05ca4628c00740000003208a6e9010000a01dd0d5a5e69b7f8f569e2fedb722c13254d7d5775a3474878e1d0190f35807d1cf"

var ext extrinsic.Extrinsic
err := codec.DecodeFromHex(encodedExt, &ext)
if err != nil {
    log.Fatal(err)
}

However, I receive the following panic:

panic: reflect: call of reflect.Value.Type on zero Value

The extrinsic can be decoded successfully using this tool: https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fasset-hub-westend-rpc.dwellir.com#/extrinsics/decode

I am using the latest master branch.