fix(range) Allow reading range from DOM when editor is disabled (#476)

Because `_readRangeFromDOM` doesn't do anything
if the editor is non-editable, the editor may get stuck with a range
that is pointing at a section that has been removed from the post.
This causes the error seen when doing the `post.walkAllLeafSections`
later in the snapshot code. The post is undefined because it is read off
the card, but the card that the range points at has been removed.

Fixes #475

src/js/editor/editor.js

@@ -395,9 +395,6 @@ class Editor {
   }
 
   _readRangeFromDOM() {
-    if (!this.isEditable) {
-      return;
-    }
     this.range = this.cursor.offsets;
   }