Custom template shown with 'Linked image not found' in File > New from template
#### Steps to reproduce:
- open Inkscape
- add a vector object into the file for easier recognition later
- File > Save template
- Add a name (other fields are not required
- Save
- now I want to use the new template: File > New from Template
- select the previously saved file
#### What happened?
The preview shows the file like this:

The commandline complains with this, when I select the template:
`SPDocument::doUnref(): invalid ref count! -1`
On Windows, the preview is just missing (all grey).
It works correctly in Inkscape 0.92.4.
#### What should have happened?
It should have used the SVG preview that we also use in other dialogs.
There is no bitmap image in my template that could be missing.
#### Inkscape Version and Operating System:
- Inkscape Version: 1.0alpha (b442a4dd31, 2019-02-17)
- Operating System: Linux Mint
- Operating System version: 19.1
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