Unable to edit gradients when object has gradient on both fill and stroke
#### Steps to reproduce:
<!-- Describe what you did (step-by-step) so we can reproduce: -->
- open Inkscape
- add new object (for example, square)
- set fill gradient to (for example) radial gradient and make some edits
- switch to stroke edit panel, set stroke to (for example) linear gradient and click on "edit gradient" button
- on appeared the gradient edit panel try to make some edits
- the first edit is applied to stroke gradient, then gradient select dropdown changes to "multiple gradients" value and does not allow to select stroke gradient from the dropdown
#### What happened?
- can't edit stroke gradient with gradient edit panel options anymore
videos with gradient edits:
https://www.youtube.com/watch?v=-GRs03Wos3s
https://www.youtube.com/watch?v=CkNyCVWY0oA
#### What should have happened?
- gradient edit panel should allow to edit gradients both for fill and for stroke
#### Inkscape Version and Operating System:
- Inkscape Version: 0.92.4
- Operating System: Ubuntu
- Operating System version: 16.04
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