The image loading code for the app cards was presuming that the icon
returned did indeed exist. In this case, it crashed due to trying to
decode a `null` image.
I noticed that when returning to the settings fragment (e.g. by closing
then reopening F-Droid while viewing), it will attempt to re-remove the
priviledged preference. This causes a crash, so we check to see that we
still have the preference before deciding to remove it.
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