rects/circles/text not exported on save as Synfig (.sif)
#### Steps to reproduce:
- Open Inkscape
- Open 
- Save as Synfig (.sif)
#### What happened?
File is blank,
```
<canvas version="0.5" width="456.322866" height="222.027092" xres="2834.645752" yres="2834.645752" view-box="-3.802691 1.850226 3.802691 -1.850226">
<name>non-paths.sif</name>
</canvas>
```
#### What should have happened?
Elements are exported correctly. I'm not sure about the correctness of the file (I don't use synfig), but something does export with Inkscape 0.92.4 (8da668285a, 2020-02-28) [non-paths92.sif](/uploads/f24a8cd724e31dff9b41c38798a23664/non-paths92.sif)
First reported in https://gitlab.com/inkscape/inbox/-/issues/2040
#### Inkscape Version and Operating System:
- 1\.1-dev (inkscape@0b728ebfcbffb9048f1b761cb44fb67f4b1a0535, 2020-03-15), extensions head at 1b5cc38a with Linux Mint 19.1
issue