Nullity check for $refs.root in modalValue watcher

[kf99916]

A title to fix the undefined error.

[codecov]

Codecov Report

Merging #254 (cdeb74f) into master (5c84f61) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #254   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files           4        4           
  Lines          82       82           
  Branches       24       24           
=======================================
  Hits           75       75           
  Misses          6        6           
  Partials        1        1           

Impacted Files	Coverage Δ
src/component.js	94.02% <100.00%> (ø)

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