Explicit model builder: Give an error if no initial state is found.

src/storm/builder/ExplicitModelBuilder.cpp

@@ -130,6 +130,7 @@ namespace storm {
             
             // Let the generator create all initial states.
             this->stateStorage.initialStateIndices = generator->getInitialStates(stateToIdCallback);
+            STORM_LOG_THROW(!this->stateStorage.initialStateIndices.empty(), storm::exceptions::WrongFormatException, "The model does not have a single initial state.");
 
             // Now explore the current state until there is no more reachable state.
             uint_fast64_t currentRowGroup = 0;