fix: Fix bug in IE11 when this.node could be null when calling onDocu…

…mentDrop (#517)

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

Closes #492

src/index.js

@@ -71,7 +71,7 @@ class Dropzone extends React.Component {
   }
 
   onDocumentDrop(evt) {
-    if (this.node.contains(evt.target)) {
+    if (this.node && this.node.contains(evt.target)) {
       // if we intercepted an event for our instance, let it propagate down to the instance's onDrop handler
       return
     }