Fix JS error when receiving reply during channel switch (#5582)

api/channel.go

@@ -772,6 +772,10 @@ func autocompleteChannels(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func viewChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 	view := model.ChannelViewFromJson(r.Body)
+	if view == nil {
+		c.SetInvalidParam("viewChannel", "channel_view")
+		return
+	}
 
 	if err := app.ViewChannel(view, c.Session.UserId, !c.Session.IsMobileApp()); err != nil {
 		c.Err = err

api/channel_test.go

@@ -903,7 +903,6 @@ func TestGetChannel(t *testing.T) {
 	if _, err := Client.GetChannels(""); err == nil {
 		t.Fatal("should have failed - wrong team id")
 	}
-
 }
 
 func TestGetMoreChannelsPage(t *testing.T) {
@@ -2003,6 +2002,10 @@ func TestViewChannel(t *testing.T) {
 		t.Log(rdata.Member.MsgCount)
 		t.Fatal("message counts don't match")
 	}
+
+	if _, err := Client.DoApiPost(Client.GetTeamRoute()+"/channels/view", "garbage"); err == nil {
+		t.Fatal("should have been an error")
+	}
 }
 
 func TestGetChannelMembersByIds(t *testing.T) {