Bug 1855289 - Do not start delayed clear active state timer if it is …

…null. r=botond

If the delayed clear active state timer is null, do not attempt to start
the timer. In the case that the delayed clear active state timer is
null, the active state should be cleared on ProcessSingleTap().

Differential Revision: https://phabricator.services.mozilla.com/D189380

gfx/layers/apz/util/ActiveElementManager.cpp

@@ -107,6 +107,10 @@ NS_IMETHODIMP DelayedClearElementActivation::GetName(nsACString& aName) {
 
 void DelayedClearElementActivation::StartTimer() {
   MOZ_ASSERT(mTimer);
+  // If the timer is null, active content state has already been cleared.
+  if (!mTimer) {
+    return;
+  }
   nsresult rv = mTimer->InitWithCallback(
       this, StaticPrefs::ui_touch_activation_duration_ms(),
       nsITimer::TYPE_ONE_SHOT);