java.lang.NullPointerException
	at org.eclipse.xtext.serializer.sequencer.AbstractSyntacticSequencer.accept(AbstractSyntacticSequencer.java:104)
	at org.eclipse.xtext.serializer.sequencer.AbstractSyntacticSequencer.navigateToAbsorber(AbstractSyntacticSequencer.java:426)
	at org.eclipse.xtext.serializer.sequencer.AbstractSyntacticSequencer.navigateToAbsorber(AbstractSyntacticSequencer.java:414)
	at org.eclipse.xtext.serializer.sequencer.AbstractSyntacticSequencer.finish(AbstractSyntacticSequencer.java:346)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.finish(SequenceFeeder.java:369)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.createSequence(BacktrackingSemanticSequencer.java:442)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.sequence_WhenUnlessExpression(AbstractCmlSemanticSequencer.java:2308)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.createSequence(AbstractCmlSemanticSequencer.java:1084)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptAction(SequenceFeeder.java:292)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.accept(SequenceFeeder.java:95)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.accept(BacktrackingSemanticSequencer.java:403)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.createSequence(BacktrackingSemanticSequencer.java:439)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.sequence_WhenUnlessExpression(AbstractCmlSemanticSequencer.java:2308)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.createSequence(AbstractCmlSemanticSequencer.java:1084)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptEObjectRuleCall(SequenceFeeder.java:299)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptRuleCall(SequenceFeeder.java:325)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.accept(SequenceFeeder.java:215)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.sequence_ConditionalExpression(AbstractCmlSemanticSequencer.java:1174)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.createSequence(AbstractCmlSemanticSequencer.java:257)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptAction(SequenceFeeder.java:292)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.accept(SequenceFeeder.java:95)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.accept(BacktrackingSemanticSequencer.java:403)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.createSequence(BacktrackingSemanticSequencer.java:439)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.sequence_AdditiveExpression(AbstractCmlSemanticSequencer.java:1097)
	at xxx.cml.serializer.AbstractCmlSemanticSequencer.createSequence(AbstractCmlSemanticSequencer.java:125)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptEObjectRuleCall(SequenceFeeder.java:299)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.acceptRuleCall(SequenceFeeder.java:325)
	at org.eclipse.xtext.serializer.acceptor.SequenceFeeder.accept(SequenceFeeder.java:222)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.accept(BacktrackingSemanticSequencer.java:401)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.createSequence(BacktrackingSemanticSequencer.java:439)

Expression
    :   ConditionalExpression
    |   WhenUnlessExpression
    |   SwitchExpression
    ;
    
ConditionalExpression
    :   'if' condition=Expression 'then' valueIfTrue=Expression 'else' valueIfFalse=Expression
    ;
    
WhenUnlessExpression returns Expression
    :   ConditionalOrExpression ({WhenUnlessExpression.value=current} operator=('when' | 'unless') condition=Expression)?
    ;
    
SwitchExpression
    :   'switch' expression=Expression '{' (caseClauses+=CaseClause)* ('default' ':' defaultExpression=Expression) '}'
    ;
    
CaseClause
    :   'case' caseExpressions+=Expression (':' 'case' caseExpressions+=Expression)* ':' valueExpression=Expression
    ;
    
ConditionalOrExpression returns Expression
    :   ConditionalAndExpression ({ConditionalOrExpression.left=current} operator='||' right=ConditionalAndExpression)*
    ;

... // left out

Primary returns Expression
    :   ParenthesizedExpression
    |   Literal
... // left out
    ;   
    
ParenthesizedExpression returns Expression
	:	'(' Expression ')'
	;