Not allow to build matrix configuration alone

Rebuilding of matrix configuration can cause problems.
Matrix project can be build if configuration is in queue (and concurrent builds is not allowed).  If it happens it causes that matrix build fails with stack trace (item in queue miss ParentBuildAction):

java.lang.NullPointerException
	at hudson.matrix.MatrixBuild$MatrixBuildExecution.doRun(MatrixBuild.java:356)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
	at hudson.model.Run.execute(Run.java:1502)
	at hudson.matrix.MatrixBuild.run(MatrixBuild.java:289)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:236)
	at hudson.model.OneOffExecutor.run(OneOffExecutor.java:66)

I think that it is better not to allow rebuild matrix configuration (in Jenkins core is not able to built matrix configuration alone -without matrix build)