util: Fix cancelEvent

Change-Id: I931d305ee70e5f8fccfed396ad6a4646089603cc
Refs: #4509
Signed-off-by: zjkmxy <bitmxy@gmail.com>
diff --git a/src/util/scheduler.cpp b/src/util/scheduler.cpp
index 9aa979e..42d7f2b 100644
--- a/src/util/scheduler.cpp
+++ b/src/util/scheduler.cpp
@@ -82,8 +82,8 @@
 {
   if (eventId != nullptr) {
     ns3::Simulator::Remove(*eventId);
-    const_cast<EventId&>(eventId).reset();
     m_events.erase(eventId);
+    const_cast<EventId&>(eventId).reset();
   }
 }