Fix crash when a sidebar is the first element in a book

[pmachapman]

Fixes #248
Port of sillsdev/machine#360

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This change is 

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.74%. Comparing base (c848f64) to head (db85e3a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #255   +/-   ##
=======================================
  Coverage   90.73%   90.74%           
=======================================
  Files         352      352           
  Lines       22315    22337   +22     
=======================================
+ Hits        20248    20270   +22     
  Misses       2067     2067           

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[ddaspit]

@ddaspit reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Enkidu93)

[Enkidu93]

@Enkidu93 reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @pmachapman)