Question

Answer: it was in the else code, but I thought there is no reason that code wouldn’t point to that line.

Can someone offer an idea how it could be possible that I get “Object reference not set to an instance of an object” on this at Invoke method:

delegate void t(tabdescriptor tab);
internal void AddItem(tabdesciptor tab)
{
    if (InvokeRequired)
    {
        t inv = new t(AddItem);
        if (inv != null && tab!= null)
            Invoke(inv, tab);
    }
    else
    {
        ....
    }
}

Answer 1

I’m not exactly sure what the actual issue is considering your example cannot be the code that executes, but please try something like this:

internal void AddItem(tabdesciptor tab)
{
    if (InvokeRequired)
    {
        Invoke(new Action<tabdescriptor>(AddItem), tab);
    }
    else
    {
        //...
    }
}

Also please make sure that it’s not actually whatever is in the else part that fails.