Fix dataTransfer.types being null crash in Safari

Fixes #155 (comment)

src/backends/HTML5.js

@@ -84,7 +84,7 @@ function createNativeDragSource(type) {
 }
 
 function matchNativeItemType(dataTransfer) {
-  const dataTransferTypes = Array.prototype.slice.call(dataTransfer.types);
+  const dataTransferTypes = Array.prototype.slice.call(dataTransfer.types || []);
 
   return Object.keys(nativeTypesConfig).filter(nativeItemType => {
     const { matchesTypes } = nativeTypesConfig[nativeItemType];
@@ -395,6 +395,15 @@ class HTML5Backend {
     } else if (nativeType) {
       // A native item (such as URL) dragged from inside the document
       this.beginDragNativeItem(nativeType);
+    } else if (
+      !dataTransfer.types && (
+        !e.target.hasAttribute ||
+        !e.target.hasAttribute('draggable')
+      )
+    ) {
+      // Looks like a Safari bug: dataTransfer.types is null, but there was no draggable.
+      // Just let it drag. It's a native type (URL or text) and will be picked up in dragenter handler.
+      return;
     } else {
       // If by this time no drag source reacted, tell browser not to drag.
       e.preventDefault();