[BUG] ManageSpaceActivity: `getExternalFilesDir` unexpectedly returned null.

2.23.0beta2

* Special case of #13207 on a nonstandard startup path

Low priority (as handled on the main startup path), but it appears in ACRA.

https://ankidroid.org/acra/app/1/bug/359712/report/ce9135f3-a1a3-45e8-b9b3-7b1f1594ff61

```
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ichi2.anki/com.ichi2.anki.ui.windows.managespace.ManageSpaceActivity}
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:4318)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4504)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:222)
at android.app.servertransaction.TransactionExecutor.executeNonLifecycleItem(TransactionExecutor.java:133)
at android.app.servertransaction.TransactionExecutor.executeTransactionItems(TransactionExecutor.java:103)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:80)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2830)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.dispatchMessage(Looper.java:315)
at android.os.Looper.loopOnce(Looper.java:251)
at android.os.Looper.loop(Looper.java:349)
at android.app.ActivityThread.main(ActivityThread.java:9041)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:593)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:929)
Caused by: z9.b: getExternalFilesDir unexpectedly returned null. Media state: mounted
at e9.r3.l(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:71)
at e9.q3.get(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:26)
at h5.k0.E(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:19)
at e9.o2.B(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:31)
at e9.j1.G(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:2)
at com.ichi2.anki.SingleFragmentActivity.onCreate(r8-map-id-9282ab195beca7114686a08db87c9a58a0224790ea7e38b9d818604305115cba:11)
at android.app.Activity.performCreate(Activity.java:9244)
at android.app.Activity.performCreate(Activity.java:9222)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1524)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:4302)
... 14 more
```