Changed GetPositionFromGuid() to return a nullabe Vector3d

Modified the two places that currently call GetPositionFromGuid() to handle the null value instead of handling an exception. This is more in line with RemoteTechologiiesGroup "codiing style" to not throw execptions where possible.
The original version of the code would of also blown up on a invalid result as there was no exception handler in place. Now it blows up with a NullRefrenceException instead of a InvalidArgumentException. The Debug.Assert does not get compiled in release mode, it is only there for testing and removing of compiler warnings.