Drag points disappear after exporting PNG

[hofstef]

Describe the bug
The drag points on activities (at the start and end of an arrow) as well as the drag points on groups (for resizing them) disappear after exporting the Domain Story as PNG file. That means that arrows become very hard to grab and rearrange; resizing groups becomes impossible.

To Reproduce
Steps to reproduce the behavior:

1. Model a sentence (actor, activity, work object)
2. Click on the arrow. Two blue drag points appear (one at the start of the arrow, one on the arrow head)
3. Click on the 'export as image' button and click PNG.
4. Click on the arrow (same as in step 2). There are no drag points visible.

Expected behavior
Drag points are visible on selected activities and groups.

Desktop (please complete the following information):

• Modeler Version: v1.2.0
• Browser: Firefox 88.0.1
• OS: Windows 10

Additional context
Exporting as SVG and downloading .dst work fine. It seems the problem only occurs after PNG download.