tabsetPanel() et al give confusing error messages when they receive incorrect input

[cpsievert]

For example:

library(shiny)

ui <- fluidPage(
    tabsetPanel(
        "1",
        tabPanel("B", "b")
    )
)

Error in divTag$attribs : $ operator is invalid for atomic vectors

This is especially confusing given that strings may be provided to navlistPanel()