fix: Cannot read property '$refs' of undefined

[iliyaZelenko]

This should solve the problem.

[codecov]

Codecov Report

Merging #273 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files          27       27           
  Lines        1030     1030           
  Branches      101      101           
=======================================
  Hits         1008     1008           
  Misses          9        9           
  Partials       13       13

Impacted Files	Coverage Δ
src/mixins/treeselectMixin.js	100% <100%> (ø)	⬆️

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[transtone]

not fixed yet (0.4.0):

TypeError: Cannot read property '$refs' of undefined
    at VueComponent.getMenu (vue-treeselect.cjs.js?ca17:1457)
    at VueComponent.onBlur (vue-treeselect.cjs.js?ca17:2252)

[arkhamvm]

not fixed yet (0.4.0):

TypeError: Cannot read property '$refs' of undefined
    at VueComponent.getMenu (vue-treeselect.cjs.js?ca17:1457)
    at VueComponent.onBlur (vue-treeselect.cjs.js?ca17:2252)

Suffers with this error too, #334 should fix it.