Add null checks on gridRect to avoid safari error

[Nikkitory]

New Pull Request

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Bug:
In Safari 17.5, moving the mouse over a line or bar chart with a datetime x axis, while it is loading, results in "TypeError null is not an object (evaluating 'r.gridRect.getBoundingClientRect')". This is referenced in this issue thread: #1790

Fix:
A basic null check.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project // not sure where to find these...
• I have performed a self-review of my own code // too hard for me to test on Mac virtual machine, but this should fix the line of code that is erroring in safari
• I have commented my code, particularly in hard-to-understand areas // I do not think additional comments are required
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works // not super relevant
• New and existing unit tests pass locally with my changes
• My branch is up to date with any changes from the main branch