If you quickly cycle between installing an app and uninstalling it, then
`app.installedApk` will still be null when AppDetails2.startUninstall()
calls InstallerService.uninstall(). It is better to crash earlier here,
before the Intent is sent with a null APK, because InstallerService is set
to receive Sticky Intents. That means they will automatically be resent
by the system until they successfully complete.
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