Are preintsol callbacks threadsafe?

I've narrowed the bug down to a print statement. When I create my master MIP (instance of XpressProblem), I create 15 binary variables (stored in Variable[] use) and one continuous variable (stored in Variable flowCost). Inside the callback, I create an instance of LinExpression using the use variables, then add a cut based on that expression and the flowCost. That seems to be fine, even with multiple threads. The exception occurs (sometimes, not always) when I try to print the cut and/or the value of flowCost.

Just now I tried to replace the original variables with those associated with the prob argument. Since I could not find a direct way to reference them, I tried prob.variableForIndex(i) as a replacement for use. That earned me a null pointer exception because "this.variableHandler" is null.

I fixed the problem by adapting your code (in your answer to another of my questions) for accessing the original names of the cut variables. This seems to work with multiple threads. Thanks for that code! It does seem to me to be unnecessarily clunky in this context. If a LinExpression instance inside a callback can use the original variables and produce correct cuts (throwing no exceptions), it seems that the toString method of that LinExpression should also work.