fix(selection): Returns previous selection if either leaf is null (#2189)

Summary:
**Summary**

Bails out early from the `getUpdatedSelectionState` method with previous selection when `anchorLeaf` or `focusLeaf` are null to prevent an error surfaced when using a decorator for emoji inserted between two spans of text entered via IME.

Thanks to robbertbrak for suggesting and cherry-picking a fix from their fork 🎉

Fixes #2187

**Test Plan**
1. Build Draft.js
1. Inject into jsfiddle reported by jdecked https://jsfiddle.net/36tvhmce/
1. Try to repro issues with Pinyin and Hiragana IME
1. Verify there is no error in the console and text is entered normally
Pull Request resolved: #2189

Reviewed By: kedromelon

Differential Revision: D17504999

Pulled By: claudiopro

fbshipit-source-id: dbd2180cf5c1af5bbe1c2b94c50767c58f524dcf

Author: Claudio Procida

meta/bundle-size-stats/Draft.js.json

@@ -28,9 +28,9 @@ function getUpdatedSelectionState(
   const selection: SelectionState = nullthrows(editorState.getSelection());
   if (__DEV__) {
     if (!anchorKey || !focusKey) {
-      /*eslint-disable no-console */
+      /* eslint-disable fb-www/no-console */
       console.warn('Invalid selection state.', arguments, editorState.toJS());
-      /*eslint-enable no-console */
+      /* eslint-enable fb-www/no-console */
       return selection;
     }
   }
@@ -47,6 +47,16 @@ function getUpdatedSelectionState(
     .getBlockTree(focusBlockKey)
     .getIn([focusPath.decoratorKey, 'leaves', focusPath.leafKey]);
 
+  if (!anchorLeaf || !focusLeaf) {
+    // If we cannot make sense of the updated selection state, stick to the current one.
+    if (__DEV__) {
+      /* eslint-disable fb-www/no-console */
+      console.warn('Invalid selection state.', arguments, editorState.toJS());
+      /* eslint-enable fb-www/no-console */
+    }
+    return selection;
+  }
+
   const anchorLeafStart: number = anchorLeaf.get('start');
   const focusLeafStart: number = focusLeaf.get('start');