Pull request: fix beta client bug

Merge in DNS/adguard-home from beta-client-bug to master

Squashed commit of the following:

commit 641f8b5eb3827e1fd9b15e60773e365206dc1f49
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Jan 11 11:54:17 2021 +0300

    home: fix beta client bug

internal/home/controlinstall.go

@@ -371,9 +371,11 @@ func (web *Web) handleInstallConfigure(w http.ResponseWriter, r *http.Request) {
 		go func() {
 			_ = web.httpServer.Shutdown(context.TODO())
 		}()
-		go func() {
-			_ = web.httpServerBeta.Shutdown(context.TODO())
-		}()
+		if web.httpServerBeta != nil {
+			go func() {
+				_ = web.httpServerBeta.Shutdown(context.TODO())
+			}()
+		}
 	}
 }