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

[pmachapman]

Fixes https://github.com/sillsdev/serval/issues/829

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This change is 

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 72.51%. Comparing base (754c97d) to head (0715464).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #360   +/-   ##
=======================================
  Coverage   72.51%   72.51%           
=======================================
  Files         423      423           
  Lines       35927    35933    +6     
  Branches     4958     4960    +2     
=======================================
+ Hits        26052    26058    +6     
  Misses       8779     8779           
  Partials     1096     1096           

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