NPE when attribute 'System' is not set in usage profile

DML solving should provide a error message with a better explanation

java.lang.NullPointerException
	at org.eclipse.emf.ecore.util.EcoreUtil.getIdentification(EcoreUtil.java:3015)
	at org.eclipse.emf.ecore.util.Diagnostician.getObjectLabel(Diagnostician.java:55)
	at org.eclipse.emf.ecore.util.Diagnostician.createDefaultDiagnostic(Diagnostician.java:88)
	at org.eclipse.emf.ecore.util.Diagnostician.validate(Diagnostician.java:107)
	at edu.kit.ipd.descartes.solving.DMLSolver.validateModel(DMLSolver.java:151)
	at edu.kit.ipd.descartes.solving.DMLSolver.validateSetup(DMLSolver.java:132)
	at edu.kit.ipd.descartes.solving.dqlconnector.PerformanceMetricsQueryConnectorImpl.startDMLSolver(PerformanceMetricsQueryConnectorImpl.java:312)
	at edu.kit.ipd.descartes.solving.dqlconnector.PerformanceMetricsQueryConnectorImpl.requestMetrics(PerformanceMetricsQueryConnectorImpl.java:214)
	at tools.descartes.dql.core.engine.interpreter.PerformanceMetricsQueryInterpreter.interpretSelectQuery(PerformanceMetricsQueryInterpreter.java:175)
	at tools.descartes.dql.core.engine.interpreter.PerformanceMetricsQueryInterpreter.interpretQuery(PerformanceMetricsQueryInterpreter.java:94)
	at tools.descartes.dql.core.engine.query.QueryExecutionEngineImpl.execute(QueryExecutionEngineImpl.java:56)
	at tools.descartes.dql.core.engine.query.QueryExecutionEngineImpl.execute(QueryExecutionEngineImpl.java:72)
	at tools.descartes.dql.core.platform.eclipse.launch.QueryExecutionEngineJob.run(QueryExecutionEngineJob.java:78)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:54)