Lock m_renderMain in Grapher::TryInitializeGraph to avoid race condition

[xuhongxu96]

Fixes #1293 .

The Calculator crashes when removing an expression with another expression just cleared.
This behavior will trigger the graph updating twice in a short time, which leads to a potential race condition to m_graph.
The app crashes because one thread is rendering the m_graph and another tries to initialize the m_graph.

Description of the changes:

• Add lock to m_renderMain->GetCriticalSection() in Grapher::TryInitializeGraph
• Minor improvements to code style

How changes were validated:

• After this fix, the bug cannot repro per steps in Graphing - remove equation crashes app #1293

As Graph is not included in the community version, tests cannot be added.

[EriWong]

Code looks good, and buddy testing does show the issue is resolved. Thanks for fixing this!