Prevent preferences fragment from crashing when resuming.

It was assuming there is always a priv ext preference. However, we
remove the priv ext preference the first time the fragment is opened. In
these circumstances, the preference no longer exists, resulting in a
NPE.

app/src/main/java/org/fdroid/fdroid/views/fragments/PreferencesFragment.java

@@ -215,6 +215,13 @@ public class PreferencesFragment extends PreferenceFragment
      */
     private void initPrivilegedInstallerPreference() {
         final CheckBoxPreference pref = (CheckBoxPreference) findPreference(Preferences.PREF_PRIVILEGED_INSTALLER);
+
+        // This code will be run each time the activity is resumed, and so we may have already removed
+        // this preference.
+        if (pref == null) {
+            return;
+        }
+
         Preferences p = Preferences.get();
         boolean enabled = p.isPrivilegedInstallerEnabled();
         boolean installed = PrivilegedInstaller.isExtensionInstalledCorrectly(getActivity())