JEXL displays an erroneous "inaccessible or unknown property" warning when you try to access a property is both accessible, and known, but that happens to return null.
Please see the attached test case. JEXL reports:
'foo.bar;' inaccessible or unknown property foo
But foo is clearly an accessible and known property because it is accessed directly 2 lines above, and 1 line above we access 'foo.baz' which works without error.
The difference seems to be that getBaz() returns "baz" whereas getBar() returns null.