Fix NPE in ParticleMathMapping when reaction has non-mass-action kinetics

`transformToStochastic` returns a `GeneralKineticsStochasticFunction`
when a reaction's rate law cannot be reduced to mass-action form (e.g.
Hill / Michaelis-Menten kinetics). The existing `instanceof
MassActionStochasticFunction` check correctly leaves `maFunc` null in
that case, but the next line dereferenced it unconditionally
(`maFunc.forwardRate()`), producing an NPE far from the real cause.

Particle math mapping requires mass-action shape regardless, so the
correct behavior is to throw the same `MappingException` whether
`maFunc` is null or has both rates null. Adding `maFunc == null ||`
to the guard yields a descriptive error pointing at the offending
reaction instead of an NPE.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jcschaff

vcell-core/src/main/java/cbit/vcell/mapping/ParticleMathMapping.java

@@ -729,7 +729,11 @@ else if(rp instanceof Product)
 				{
 					maFunc = (MassActionStochasticFunction)stochasticFunction;
 				}
-				if ((maFunc.forwardRate() == null && maFunc.reverseRate() == null))
+				// transformToStochastic falls back to GeneralKineticsStochasticFunction
+				// when the rate law can't be reduced to mass-action form (e.g. Hill /
+				// Michaelis-Menten kinetics). Particle math mapping requires mass-action
+				// shape, so report the same error in either case rather than NPE.
+				if (maFunc == null || (maFunc.forwardRate() == null && maFunc.reverseRate() == null))
 				{
 					throw new MappingException("Cannot generate stochastic math mapping for the reaction:" + reactionStep.getName() + "\nLooking for the rate function according to the form of k1*Reactant1^Stoir1*Reactant2^Stoir2...-k2*Product1^Stoip1*Product2^Stoip2.");
 				}