[#749] morph: Fix panic when closing morph client

Signed-off-by: Anton Nikiforov <an.nikiforov@yadro.com>

pkg/morph/client/notifications.go

@@ -16,7 +16,10 @@ func (c *Client) Close() {
 	// closing should be done via the channel
 	// to prevent switching to another RPC node
 	// in the notification loop
-	close(c.closeChan)
+	if c.closed.CompareAndSwap(false, true) {
+		close(c.closeChan)
+	}
+	c.wg.Wait()
 }
 
 // ReceiveExecutionNotifications performs subscription for notifications