editor: set_syncstring → to_str: doc must be set

[haraldschilly]

for a .tex file:

chrome: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.141 Safari/537.36

smc_git_rev: 8af2de1

Date: 2021-01-13 16:17:35.456599

doc must be set

Error: doc must be set
    at t.to_str (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:672:6366)
    at n.t.set_syncstring (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:101:22994)
    at n.t.set_value (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:101:22770)
    at eval (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:158:6453)
    at eval (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:99226)
    at t.unstable_runWithPriority (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:135597)
    at qa (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:59809)
    at cs (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:99148)
    at ps (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:101299)
    at eval (eval at <anonymous> (https://cocalc.com/app:1:8369), <anonymous>:1309:114974)