Add check for backstack size before pushing DownloadController (#5312)

(cherry picked from commit 64f95af3e51bc721db1be137e34007e72ee4aeb9)

app/src/main/java/eu/kanade/tachiyomi/ui/main/MainActivity.kt

@@ -198,10 +198,12 @@ class MainActivity : BaseViewBindingActivity<MainActivityBinding>() {
                         controller?.showSettingsSheet()
                     }
                     R.id.nav_updates -> {
+                        if (router.backstackSize == 1) {
                             router.pushController(DownloadController().withFadeTransaction())
                         }
                     }
                 }
+            }
             true
         }