It is probably an internal jBASE error and cannot theoretically be caused by any form of BASIC programming.
If you have any 'C' code written outside of the standard jBASE then it could be that that is in error. If not, then it is a jBASE error and you'll need to report it to jBASE support.
The error comes from programming in C. Often in C you need some blocks of memory to do various tasks and these are taken off the heap space using a function called malloc(). When you're finished with them, in 'C' you call a function called free() to release that memory back to the free space pool. What the error is saying is that the call to free() has been given a pointer that points to address 0 (aka NULL pointer) which is an illegal address.
Greg