TYPO3 Core

Steps to reproduce the issue (I tried with the latest master, but I guess it is the same with 7.6.x):

1) add CE text w/media
2) edit the HTML and add an image from an external url e.g. writing: (*)

<img src="http://www.serateotaku.it/img/jeeg/jeeg1.jpg" alt="" />

3) exit the source code mode; click on the image and click on the image button to edit it
4) you will obtain the error:

Oops, an error occurred!
The UID of file has to be numeric. UID given: "null" 
More information regarding this error might be available online.

(*) you can even drag and drop the image from another window of the browser directly into the RTE area

Explanation
this is because an images added with the RTE interface have the attribute: data-htmlarea-file-uid="XX" while the "external" image obviously lacks it.

the behavior is understandable, and images should not be added that way, but I think that a less scary error message would be more appropriate.

something like
"The file you have selected doesn't seem to be found among the assets managed by the CMS, so its properties can't be edited. please click here to select another file etc etc..."