It is my understanding that "connect" code placement is not critical
No, not true. You must connect the signal and slot before any of the signals you want to process can be issued. The customary place to make the connection is in the constructor of the parent of the two widgets you want to connect. This ensures that the connection is made only once and before either of the connected widgets can do anything. There are other places, but this is the most common.

RTFM implies that passing null pointer may be the problem...
Almost always either a null pointer or a pointer that has not been initialized. If you are passing a pointer in to your secondary dialog via a function call, then calling connect() using the variable that holds that pointer before you have made that call (i.e. in the dialog constructor) means you are using an uninitialized pointer.