rbind empty ordered factor triggers savetl_init errors

[shrektan]

See the example below I'm using the latest dev version of data.table on Windows.

library(data.table)
x <- data.table(A = character(), B = double())
x[, A := ordered(A, levels = character())]
rbind(x, x)
#> Error in rbindlist(l, use.names, fill, idcol): STRING_PTR() can only be applied to a 'character', not a 'NULL'
rbind(x, x)
#> Error in rbindlist(l, use.names, fill, idcol): Internal error: savetl_init checks failed (0 100 0x0000000021484990 0x000000001f6b7530). please report to data.table issue tracker.

^{Created on 2020-11-03 by the reprex package (v0.3.0)}

Session info

devtools::session_info()
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#>  ctype    Chinese (Simplified)_China.936
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#>  date     2020-11-03                    
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