Don't assume iface is available

[nyalldawson]

E.g. in qgis_process iface is None

Fixes #38862

[alexbruy]

Oh, you are faster. I was about to submit fix for this. But your approach looks better than mine.

[nyalldawson]

@alexbruy it's a temporary fix -- for 3.18 i'd thinking of pulling apart processing into multiple separate components/plugins:

• qgis python algs
• gdal provider
• saga provider
  -grass provider
  -otb provider
• processing "core" (the remaining python bits required for processing to run, e.g. model provider + script provider)
• processing "gui" -- all the stuff which requires iface, such as the toolbox, model designer, etc.

The intention then would be that for 4.0 we'd drop processing "core" (it'd all be in c++), and if possible at that stage processing "gui" too.

What do you think?

[alexbruy]

I'm fully agree that Processing should become a core component of QGIS. Only providers will remain plugin. So I'm a big +1 here.

[m-kuhn]

At the same time, could we update plugin init methods to

• initGui() called by app
• initCore() called by app, process, server