Error on image properties

[slacto]

We get this error in certain situations when we want to edit image properties:

• If you first select the image (e.g. by left-clicking on it) and then right-click and select image properties, it works fine.
• If you have not marked the image before right-clicking on it, the error will appear.
• If you use the "Image properties" button in the toolbar it will work in all cases (because the "Image properties" button is only visible when the image is selected)