be more defensive in QPixel#preference about null values from _getPreference

Author: Oaphi

app/assets/javascripts/qpixel_api.js

@@ -217,10 +217,10 @@ window.QPixel = {
     }
 
     let prefs = await QPixel._getPreferences();
-    let value = community ? prefs.community[name] : prefs.global[name];
+    let value = community ? prefs?.community[name] : prefs?.global[name];
 
     // Note that null is a valid value for a preference, but undefined means we haven't fetched it.
-    if (typeof (value) !== 'undefined') {
+    if (typeof value !== 'undefined') {
       return value;
     }
     // If we haven't fetched a preference, that probably means it's new - run a full re-fetch.

global.d.ts

@@ -162,9 +162,9 @@ interface QPixel {
   /**
    * Get an object containing the current user's preferences. Loads, in order of precedence, from local variable,
    * {@link localStorage}, or Redis via AJAX.
-   * @returns JSON object containing user preferences
+   * @returns user preferences or `null` on failure
    */
-  _getPreferences?: () => Promise<UserPreferences>;
+  _getPreferences?: () => Promise<UserPreferences | null>;
 
   /**
    * Get the key to use for storing user preferences in localStorage, to avoid conflating users