Any variable value is null until a sub or function sets it to a value. That will remain its value until it is reset by code, or when the function stops running.
Modules are not 'running' when the app opens until they are called.
Place a breakpoint on the code in the module, and step thru the code when the form opens. Do it again when the other method is called and you should be able to determine why it is changing the value of the variable.