Fixes crash which sometimes happens in PieChartRow

[harryblam]

Description

We experience random crashes sometimes when the PieChartRow data is mutated often (see below screenshot).

Motivation and Context

• This code makes the race condition that was causing the crash not possible.
• It also fixes the compile error i'm getting on Xcode 15

How Has This Been Tested?

Screenshots (if appropriate):

Crash when the slices index was out of range because the data was mutated whilst getting it.

Types of changes

• [x] 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)
• [ ] Non-functional change (Updating Documentation, CI automation, etc..)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.