attempt to use non-existent nimbleList not cleanly error trapped

[paciorek]

mynf <- nimbleFunction(
    setup = function() {
        myList <- nimbleList(x = double())
    },
    run = function() {
        ll <- myList$new()
        branch$x = 3.0  # misnamed
    }
)

rmynf <- mynf()
cmynf <- compileNimble(rmynf)
# Error in if (!(isSymFunc || isSymList)) stop(exprClassProcessingErrorMsg(code,  : 
#  missing value where TRUE/FALSE needed