fix Jtk_RemoveTimer()

src/gui/jtk/Jtk_timer.cc

@@ -64,9 +64,9 @@ void Jtk_RemoveTimer(size_t timer_id)
 {
     if (timer_id < g_timers.size())
     {
-        g_timers[timer_id] = nullptr;
         auto timer = g_timers[timer_id];
         g_active_timers.remove(timer);
+        g_timers[timer_id] = nullptr;
     }
 }