Fix Darwin crash with wildcard subscription. (#15941)

We were deleting things on the wrong work queue, so ended up deleting
an object while we were in the middle of executing one of its methods.

The fix is to make sure to delete Matter objects on the Matter work queue.

src/darwin/Framework/CHIP/CHIPDevice.mm

@@ -1298,7 +1298,12 @@ - (instancetype)initWithPath:(const ConcreteDataAttributePath &)path value:(null
     dispatch_async(mQueue, ^{
         callback(nil, err);
 
-        delete myself;
+        // Deletion of our ReadClient (and hence of ourselves, since the
+        // ReadClient has a pointer to us) needs to happen on the Matter work
+        // queue.
+        dispatch_async(DeviceLayer::PlatformMgrImpl().GetWorkQueue(), ^{
+            delete myself;
+        });
     });
 
     mHaveQueuedDeletion = true;