Bug 1789396: Null check acc before calling AsLocal() in RemoteAccessibleBase::BoundsWithOffset. r=morgan

We expect acc to be a local OuterDocAccessible.
However, the OuterDocAccessible might die before the top level DocAccessibleParent, in which case acc will be null.
In this case, the DocAccessibleParent is about to die anyway, so it doesn't matter that the bounds we return are slightly wrong (since they can't take the OuterDoc into account).

Differential Revision: https://phabricator.services.mozilla.com/D156773

accessible/ipc/RemoteAccessibleBase.cpp

@@ -614,7 +614,10 @@ LayoutDeviceIntRect RemoteAccessibleBase<Derived>::BoundsWithOffset(
     // This block is not thread safe because it queries a LocalAccessible.
     // It is also not needed in Android since the only local accessible is
     // the outer doc browser that has an offset of 0.
-    if (LocalAccessible* localAcc = const_cast<Accessible*>(acc)->AsLocal()) {
+    // acc could be null if the OuterDocAccessible died before the top level
+    // DocAccessibleParent.
+    if (LocalAccessible* localAcc =
+            acc ? const_cast<Accessible*>(acc)->AsLocal() : nullptr) {
       // LocalAccessible::Bounds returns screen-relative bounds in
       // dev pixels.
       LayoutDeviceIntRect localBounds = localAcc->Bounds();