Mail: Mitigate constant access to the nullable fields (`folder` & `store`) in `AbstractMailReceiver`

[mjd507]

both folder & store fields are nullable in AbstractMailReceiver.

since we are going to apply jspecify for nullability check, in order to minimize the scope of impact,

I want to mitigate them with local variables instead of constant access.

so it will be clear for me when it's null, when it's not null

[artembilan]

So, what is the goal of this issue?
Looks like it fully depends on the #10096 .

Do you expect that to be merged first?

[mjd507]

hmm, I plan to fix #10100 first, then this one, then #10096