added check for reverse reactions being null in net reactions block
added check for reverse reactions being null in net reactions block

cf. similar edit in 79d632801d097324b86621f32b04167ccb7ee60c for kleak block
thanks to Shamel for finding and reporting the NPE

nyee/RMG-Java,jwallen/RMG-Java,KEHANG/RMG-Java,KEHANG/RMG-Java,nyee/RMG-Java,jwallen/RMG-Java,jwallen/RMG-Java,enochd/RMG-Java,keceli/RMG-Java,ReactionMechanismGenerator/RMG-Java,faribas/RMG-Java,KEHANG/RMG-Java,rwest/RMG-Java,ReactionMechanismGenerator/RMG-Java,KEHANG/RMG-Java,enochd/RMG-Java,jwallen/RMG-Java,enochd/RMG-Java,connie/RMG-Java,keceli/RMG-Java,KEHANG/RMG-Java,keceli/RMG-Java,rwest/RMG-Java,rwest/RMG-Java,keceli/RMG-Java,rwest/RMG-Java,nyee/RMG-Java,ReactionMechanismGenerator/RMG-Java,nyee/RMG-Java,enochd/RMG-Java,rwest/RMG-Java,faribas/RMG-Java,ReactionMechanismGenerator/RMG-Java,enochd/RMG-Java,connie/RMG-Java,KEHANG/RMG-Java,connie/RMG-Java,jwallen/RMG-Java,faribas/RMG-Java,enochd/RMG-Java,jwallen/RMG-Java,keceli/RMG-Java,faribas/RMG-Java,nyee/RMG-Java,keceli/RMG-Java,connie/RMG-Java,nyee/RMG-Java,ReactionMechanismGenerator/RMG-Java,connie/RMG-Java