Some bug fixes

[tools400]

Pull Request

This pull request fixes some problems that I noticed when learning flutter_reactive_ble.

Preparation

Steps I took for launching the app:

• Cloned the project to my Windows PC
• Executed command flutter config --jdk-dir="c:\Program Files\Java\jdk-17 for setting the Java version
• Executed command flutter pub get for resolving dependencies

Fix problem that app cannot be started

Commit: Update SDK version for starting the app.

• Select Start Debugging (VSCode) to launch the app
• Error: FAILURE: Build failed with an exception.

Example error message:

Dependency 'androidx.fragment:fragment:1.7.1' requires libraries and applications that
           depend on it to compile against version 34 or later of the
           Android APIs.

Fix error on LateInitializationError exception

Commit: Fix 'LateInitializationError' exception.

Steps to reproduce the problem:

• Start App
• Start Scan
• Select any device
• Tap the 'back' arrow to return to the device list
• Error: LateError (LateInitializationError: Field '_connection@141363709' has not been initialized.)

Fix problem that characteristics cannot be expanded

Commit: Fix expanding characteristics.

Steps to reproduce the problem:

• Start App
• Start Scan
• Select any device
• Tap the Connect buton
• Tap the Discover Services button
• Tap the down arrow on a characteristics card to expand the card
• Error: card is not expanded

Fix another error on LateInitializationError exception

Commit: Fix another 'LateInitializationError' exception.

Steps to reproduce the problem:

• Start App
• Start Scan
• Select any device
• Tap the Discover Services button
• Tap the down arrow on a characteristics card to expand the card
• Tap the characteristics card to open the Select an operation dialog
• Tap the close button
• Error: LateError (LateInitializationError: Field 'subscribeStream' has not been initialized.)

[Taym95]

I will close this, since #889 just got merged and contains these chnages