kivy 2.1 seemingly became more sensitive to threading issues.
This used to work on kivy 2.0 and older, but 2.1 is complaining.
```
I | paymentrequest | Error while contacting payment URL: https://bitpay.com/i/... -- error type: <class 'aiohttp.client_exceptions.ClientResponseError'> -- Got HTTP status code 404.
E | util | Exception in get_payment_request: TypeError('Cannot create graphics instruction outside the main Kivy thread')
Traceback (most recent call last):
File "/home/user/wspace/electrum/electrum/util.py", line 1175, in wrapper
return await func(*args, **kwargs)
File "/home/user/wspace/electrum/electrum/util.py", line 1033, in get_payment_request
on_pr(request)
File "/home/user/wspace/electrum/electrum/gui/kivy/main_window.py", line 460, in on_pr
self.show_error("invoice error:" + pr.error)
File "/home/user/wspace/electrum/electrum/gui/kivy/main_window.py", line 1066, in show_error
self.show_info_bubble(text=error, icon=icon, width=width,
File "/home/user/wspace/electrum/electrum/gui/kivy/main_window.py", line 1092, in show_info_bubble
info_bubble = self.info_bubble = Factory.InfoBubble()
File "/usr/local/lib/python3.8/dist-packages/kivy/uix/bubble.py", line 199, in __init__
self._arrow_layout = BoxLayout()
File "/usr/local/lib/python3.8/dist-packages/kivy/uix/boxlayout.py", line 145, in __init__
super(BoxLayout, self).__init__(**kwargs)
File "/usr/local/lib/python3.8/dist-packages/kivy/uix/layout.py", line 76, in __init__
super(Layout, self).__init__(**kwargs)
File "/usr/local/lib/python3.8/dist-packages/kivy/uix/widget.py", line 361, in __init__
self.canvas = Canvas(opacity=self.opacity)
File "kivy/graphics/instructions.pyx", line 608, in kivy.graphics.instructions.Canvas.__init__
File "kivy/graphics/instructions.pyx", line 154, in kivy.graphics.instructions.InstructionGroup.__init__
File "kivy/graphics/instructions.pyx", line 60, in kivy.graphics.instructions.Instruction.__init__
TypeError: Cannot create graphics instruction outside the main Kivy thread
```
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