Accessing a widget from a thread

I’m a novice in QNX Dev.

I use the PhAB and when I create a thread from the Window Create Event (A
thread used to read data from a socket port) I would like to display
received text in the ListBox used in my app (the Window contain the ListBox)
My Application is simple but I was believing that all vars were shared by
threads in the same process…
When I try to write the text in the list Box It crashes (but the writing
method is correct) …
Please let me know how to make accessible widgets in read/write from threads
or let me know where is my error…

(I’ve started QNX since 1 week and I’m very surprised by it’s power and
versatile purpose )
Thanks for all