summary.panel_data's fall-back does not work

[ghost]

When pkg skimr is not installed, the fall-back fails:

data("WageData")
> wages <- panel_data(WageData, id = id, wave = t)
> summary(wages, lwage, exp, wks)
Loading required namespace: skimr
Failed with error:  ‘there is no package called ‘skimr’’
Get better summaries of panel_data frames by installing the skimr package. Falling back to default summary.data.frame...
Error in UseMethod("select_") : 
  no applicable method for 'select_' applied to an object of class "function"