Fix error that occur when clicking "insert image" button

If "element.childNodes" is empty, "element.tagName" cause "TypeError: Cannot read property 'tagName' of undefined".
This problem may occurs in clicking "insert image" button or "insert table" button, when textarea is empty. (Mac OSX 10.9.4, Chrome 35.0.1916.153)
Merge pull request #374 from ko-suga/patch-1

Fix error that occur when clicking "insert image" button