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

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

@@ -172,7 +172,9 @@ class MainActivity : BaseViewBindingActivity<MainActivityBinding>() {
                         controller?.showSettingsSheet()
                     }
                     R.id.nav_updates -> {
-                        router.pushController(DownloadController().withFadeTransaction())
+                        if (router.backstackSize == 1) {
+                            router.pushController(DownloadController().withFadeTransaction())
+                        }
                     }
                 }
             }