The default MyFaces Error handling throws a NullPointerException during the writing out of the component tree when an attribute's ValueExpression.getExpressionString() returns null.
As an example, this can happen with Richfaces 4.1.x skinning as the org.richfaces.application.GlobalResourcesViewHandler$SkinningResourceRenderedExpression
used for "rendered" on the __rf_skinning_resource UIOutput has a null value for getExpressionString().
From what I can tell, it's legal for Expression.getExpressionString() to return null.
In any case, since this is an error handling routine, it should
probably be more flexible in handling unexpected values.
What do we want to do in this case?
We could skip outputting the attribute.
We could output the attribute with empty string as the value expression.
In this particular case, we could check for isReadOnly() and if true, output getValue(). Not sure if we'd want to do that in the case of !isReadOnly(). In fact, I'm not entirely certain we want to be evaluating getValue().
We could output the class of the value expression.
As a first pass, I am going to output the value expression as the empty string.