tree wide: check if build context is still valid after async call

tree wide: check if build context is still valid after async call

> Storing BuildContext for later usage can easily lead to difficult
> to diagnose crashes. Asynchronous gaps are implicitly storing
> BuildContext and are some of the easiest to overlook when writing
> code.
>
> When a BuildContext is used, a mounted property must be checked
> after an asynchronous gap, depending on how the BuildContext is
> accessed:
>
> * When using a State's context property, the State's mounted
>   property must be checked.
> * For other BuildContext instances (like a local variable or
>   function argument), the BuildContext's mounted property must be
>   checked.

-- https://dart.dev/tools/linter-rules/use_build_context_synchronously

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>