Update Ref to support initialization and assign optional value

[yantaozhao]

Description

Support initialize Ref with value directly, and allow re-assign None for intent clarify if needed.

Example code

In some scenarios, one control was created without known ref or the ref unavailable, but will be referenced in future.
It is more concise to direct initialize a ref with value rather than in a separate assignment statement.

# factory method to create and return a flet control
def control_factory(control_type: str) -> ft.Control:
    match control_type:
        case pattern_1:
            return ft.TextField(...)  # no ref here
        case pattern_2:
            return ft.Switch(...)
        case pattern_3:
            return ft.Slider(...)
        case ...:
            ......

controls: list[ft.Ref] = []  # list of ref to use later
control: ft.Control = control_factory(...)  # <-- create control in factory, with no ref passing into factory
# control_ref = ft.Ref()
# control_ref.current = control
control_ref = ft.Ref(control)  # <-- initialize directly instead of passing ref variable into factory or assigning it in another statement
controls.append(control_ref)

......
controls[i].current = None  # <-- later, no need to ref to control, but keep ref itself for some reason

Type of change

• Bug fix: TypeError: cannot create weak reference to 'NoneType' object when passing None to setter.
• New feature: initialize Ref

Summary by Sourcery

Enable direct Ref initialization with a value and optional None assignment to explicitly clear the reference

New Features:

• Allow initializing Ref with an optional initial value via constructor
• Enable clearing a Ref by assigning None to its current property

[ndonkoHenri]

Thanks for your contribution. Will be reviewed when possible.