`$<-` can create invalid tsibbles

[mitchelloharawild]

library(tsibble)
pedestrian$Sensor <- NULL
pedestrian$Date_Time <- NULL
pedestrian
#> # A tsibble: 66,037 x 3 [1h]
#> # Key:       Sensor [4]
#>    Date        Time Count
#>    <date>     <int> <int>
#>  1 2015-01-01     0  1630
#>  2 2015-01-01     1   826
#>  3 2015-01-01     2   567
#>  4 2015-01-01     3   264
#>  5 2015-01-01     4   139
#>  6 2015-01-01     5    77
#>  7 2015-01-01     6    44
#>  8 2015-01-01     7    56
#>  9 2015-01-01     8   113
#> 10 2015-01-01     9   166
#> # … with 66,027 more rows

^{Created on 2020-05-19 by the reprex package (v0.3.0)}