beacon: add nil check for AccessEvents (#453)

If Isthmus is activated, we attempt to add the L2ToL1MessagePasser
to state access events. Check that AccessEvents exists before attempting
to AddAccount

Co-authored-by: Vinod Damle <5338861+vdamle@users.noreply.github.com>

consensus/beacon/consensus.go

@@ -410,7 +410,10 @@ func (beacon *Beacon) FinalizeAndAssemble(chain consensus.ChainHeaderReader, hea
 		// State-root has just been computed, we can get an accurate storage-root now.
 		h := state.GetStorageRoot(params.OptimismL2ToL1MessagePasser)
 		header.WithdrawalsHash = &h
-		state.AccessEvents().AddAccount(params.OptimismL2ToL1MessagePasser, false) // include in execution witness
+		sa := state.AccessEvents()
+		if sa != nil {
+			sa.AddAccount(params.OptimismL2ToL1MessagePasser, false) // include in execution witness
+		}
 	}
 
 	// Assemble the final block.