[Core][Live Preview]: It will pop up Live Preview Error dialog if launch live preview with an image.

[julieyuan]

Steps:

1. Launch Brackets and open the default sample folder.
2. Open the image quick-edit.png from the screenshots folder.
3. Click live preview icon on the right sidebar.(It will pop up an error dialog and preview index.html in Chrome)
4. Close the error dialog
5. Close the webpage from Chrome.

Results:
At step3, it pops up Live Preview Error dialog and shows the message "Unable to load Live Development page". But the index.html page is opened and previewing in Chrome. But the live preview icon doesn't change to orange.
At step5, the live preview icon will show the tooltip.

Expected:
This dialog should not appear and the live preview icon should show as connected status.

ENV: MAC 10.8 and Win8.1 English OS
Build: 0.33.0-10155

Snapshots:
Please refer to snapshot for details:
Step3:

Step5:

[marcelgerber]

This only happens when the Working Files are not empty.

[marcelgerber]

It is caused by returning null by _getCurrentDocument() on line 571. Should we better use the return value of _getInitialDocFromCurrent?

[njx]

Medium priority to @couzteau who is reviewing @SAplayer's fix.

[julieyuan]

This issue is fixed in build 0.34.0-10270. Thanks and closing it. Here is snapshot for reference: