fix: Add null check for node in onDocumentDrop

[pcgilday]

What kind of change does this PR introduce?

• bugfix

If relevant, did you update the documentation?

• Yes, I've updated the documentation
• Not relevant

Summary

• Protect calling this.node.contains when this.node is null
• Fix IE11 bug where onDocumentDrop gets called with null this.node ref

closes #492

[codecov]

Codecov Report

Merging #517 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #517   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files           3        3           
  Lines         197      197           
  Branches       51       51           
=======================================
  Hits          191      191           
  Misses          6        6

Impacted Files	Coverage Δ
src/index.js	97.07% <100%> (ø)	⬆️

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[okonet]

Thanks for working on it!