Bug 1811332 - Make sure `this.indicator` is truthy before using it. r=Gijs

The `indicator` getter may return `null` so we cannot just call methods on it.

Differential Revision: https://phabricator.services.mozilla.com/D169717

browser/components/downloads/content/indicator.js

@@ -471,7 +471,11 @@ const DownloadsIndicatorView = {
   },
 
   _maybeScheduleProgressUpdate() {
-    if (!this._progressRaf && document.visibilityState == "visible") {
+    if (
+      this.indicator &&
+      !this._progressRaf &&
+      document.visibilityState == "visible"
+    ) {
       this._progressRaf = requestAnimationFrame(() => {
         // indeterminate downloads (unknown content-length) will show up as aValue = 0
         if (this._percentComplete >= 0) {