blockchain/v2: allow setting nil switch, for CustomReactors()

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Fixes an issue reported in https://github.com/tendermint/tendermint/issues/4595#issuecomment-612667441.

Not sure if this is sufficient to fully remove the reactor, but it fixes the immediate problem.
______

For contributor use:

- [x] Wrote tests
- [x] ~Updated CHANGELOG_PENDING.md~
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] ~Updated relevant documentation (`docs/`) and code comments~
- [x] Re-reviewed `Files changed` in the Github PR explorer

blockchain/v2/reactor.go

@@ -187,12 +187,12 @@ func NewBlockchainReactor(
 
 // SetSwitch implements Reactor interface.
 func (r *BlockchainReactor) SetSwitch(sw *p2p.Switch) {
-	if sw == nil {
-		panic("set nil switch")
-	}
-
 	r.Switch = sw
-	r.io = newSwitchIo(sw)
+	if sw != nil {
+		r.io = newSwitchIo(sw)
+	} else {
+		r.io = nil
+	}
 }
 
 func (r *BlockchainReactor) setMaxPeerHeight(height int64) {

blockchain/v2/reactor_test.go

@@ -411,6 +411,22 @@ func TestReactorHelperMode(t *testing.T) {
 	}
 }
 
+func TestReactorSetSwitchNil(t *testing.T) {
+	config := cfg.ResetTestRoot("blockchain_reactor_v2_test")
+	defer os.RemoveAll(config.RootDir)
+	genDoc, privVals := randGenesisDoc(config.ChainID(), 1, false, 30)
+
+	reactor := newTestReactor(testReactorParams{
+		logger:   log.TestingLogger(),
+		genDoc:   genDoc,
+		privVals: privVals,
+	})
+	reactor.SetSwitch(nil)
+
+	assert.Nil(t, reactor.Switch)
+	assert.Nil(t, reactor.io)
+}
+
 //----------------------------------------------
 // utility funcs