Add lazy initialization to SerialController

[josesimoes]

Description

• Add lazy initialization to SerialController.

Motivation and Context

• Make sure that the variables do exist when being used. Using the previous code was causing issues because on occasions the types could still be unresolved.
• Addresses GPIO NULL Reference Home#383.

How Has This Been Tested?

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Signed-off-by: José Simões jose.simoes@eclo.solutions

[nfbot]

Hi @josesimoes,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉

[steffalk]

Looks fine. If lock() is expensive, you could extend the pattern to: if (s_instance != null) {return s_instance;} else {… the code you provided so far, that is the lock() {...} block). That way, if many clients want to have the instance, unnecessary lock() operations would be saved.