fixed wrong usage of boost::variant in NextStateGenerator

Former-commit-id: 0ba0e8d565

src/generator/NextStateGenerator.cpp

@@ -30,7 +30,7 @@ namespace storm {
         }
         
         std::string const& LabelOrExpression::getLabel() const {
-            return boost::get<std::string const&>(labelOrExpression);
+            return boost::get<std::string>(labelOrExpression);
         }
         
         bool LabelOrExpression::isExpression() const {
@@ -38,7 +38,7 @@ namespace storm {
         }
         
         storm::expressions::Expression const& LabelOrExpression::getExpression() const {
-            return boost::get<storm::expressions::Expression const&>(labelOrExpression);
+            return boost::get<storm::expressions::Expression>(labelOrExpression);
         }
         
         NextStateGeneratorOptions::NextStateGeneratorOptions(bool buildAllRewardModels, bool buildAllLabels) : buildAllRewardModels(buildAllRewardModels), buildAllLabels(buildAllLabels), buildChoiceLabels(false) {