Runtime error on theme change when built with -tags mobile

[Jacalz]

Describe the bug:

An application that tries to set the theme before a window is created will fail due to an error on run-time when built using -tags mobile, but not if it is built without that tag. The errors that are getting spewed out are the following:

 panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x69201a]goroutine 8 [running]:
fyne.io/fyne/internal/app.ApplyThemeTo(0x0, 0x0, 0x895ae0, 0xc000124000)
	/home/jacob/go/src/fyne.io/fyne/internal/app/theme.go:23 +0xfa
fyne.io/fyne/internal/app.ApplySettings(0x890760, 0xc000120000, 0x894480, 0xc00006a360)
	/home/jacob/go/src/fyne.io/fyne/internal/app/theme.go:30 +0xf0
fyne.io/fyne/app.NewAppWithDriver.func1(0xc000026480, 0xc00006a360)
	/home/jacob/go/src/fyne.io/fyne/app/app.go:108 +0x6b
created by fyne.io/fyne/app.NewAppWithDriver
	/home/jacob/go/src/fyne.io/fyne/app/app.go:105 +0x15f

To Reproduce:

Steps to reproduce the behaviour:

1. Build and run this code with go run.
2. Build and run the same code snippet with go run -tags mobile (will not start at all).
3. Instead build and run this code snippet with go run -tags mobile.

Example code:

Not working code version: https://gist.github.com/Jacalz/b09b4cceca7bdc6b7946fd44fdf55b68
Working code version: https://gist.github.com/Jacalz/723b828fbbb3fce991eb57cd5d20e52b

Device (please complete the following information):

• OS: Linux
• Version: 5.5.3
• Go version: 1.13.6
• Fyne version: 1.2.0, 1.2.2 and devlop branch have all been tested.

Currently tested on Linux only.

[andydotxyz]

on develop for testing, this will merge into pending v1.2.3 release shortly

[andydotxyz]

Now on the pending 1.2.3 release branch