Widget not available on developer iPhone
This may not be related to you having two different versions. One thing that an Apple engineer at WWDC pointed out to me is any time you do a new install of an app, you have to manually launch that app at least once before they make its widgets available in the widget browser. This apparently does not include launching it from Xcode (which they said is an issue that they are working on.)
As such, in our case, when debugging our app, we had to minimize (i.e. go back to the home screen) the app, then manually tap on the icon to bring it right back to the front. Once we did that, our widgets would show up in the browser again.
Again, make sure to minimize, not close the app or you'll kill your debug session. Minimize, tap the icon to open it back up again and you should be good to go.
Their reasoning is they only want things to appear in the browser that you have explicitly accessed, not simply installed, and since you're basically doing new installs throughout the dev process, that can cause the issue.