improve ContextManager

[bollhals]

This PR improves the performance of the ContextManager and enables nullable reference types for this file.
(Info: Nullable reference types only make a difference where it is enabled, so this is safe to do)

PR with parts of #2373

Types of changes

• 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 not work as expected).
• Performance improvement
• Refactoring (so no functional change)
• Other (docs, build config, etc)

Checklist:

• I've added tests for my code. (most of the time mandatory)
• I have added an entry to the changelog. (mandatory)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.