The NullReferenceException in HUDManager.cs occurs because itemdata is null. This means there is no CItemIndicator component on the HUDManager's GameObject. Is it on a child GameObject? If so, use GetComponentInChildren.
For the second error (Lua reporting that ShowAlert() is not a registered method), please temporarily set the Dialogue Manager's Other Settings > Debug Log to Info. Then reproduce the problem.
When the Dialogue Manager starts, the Console window should log:
Dialogue System: Registering Lua function ShowAlert
Make sure you see this message. Then review the logs after that for any occurrences of:
Dialogue System: Unregistering Lua function ShowAlert
If you see any occurrence, click on it to view the stack trace in the bottom part of the Console log. Review it (or post a screenshot here) to determine what's unregistering it.
At any time, please feel free to send a reproduction project to tony (at) pixelcrushers.com along with reproduction steps.
How to Stack Alert messages
Re: How to Stack Alert messages
I was able to solve the problem by reinforcing my script by looking at your explanation.
Thank you so much for your support, Tony!
Re: How to Stack Alert messages
Happy to help! I'm glad you got it to work the way you want.