gui: Add null check and print warning in DbNetDescriptor::highlight()

[openroad-ci]

Fixes #9383

• Note that this PR added a defensive to avoid the crash. The root-cause that produces the null net should be found and fixed in another PR.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[gemini-code-assist]

Code Review

This pull request adds a defensive null check in DbNetDescriptor::highlight() to prevent a crash when the net is null, which is a good safety measure. My review includes suggestions to improve how the logger is accessed, promoting better encapsulation and reducing dependencies by using an existing class member instead of a global singleton.

[gemini-code-assist]

With the suggested change to use sta_->getLogger(), this include is no longer necessary and can be removed to reduce dependencies.

[jhkim-pii]

sta_->getLogger() does not exist.

[gemini-code-assist]

The logger can be obtained more directly via the sta_ member, which is already available in this class. This avoids using the global OpenRoad singleton and including OpenRoad.hh.

Suggested change

	utl::Logger* logger = ord::OpenRoad::openRoad()->getLogger();
	logger->warn(
	utl::GUI, 111, "Net is nullptr in DbNetDescriptor::highlight().");
	sta_->getLogger()->warn(utl::GUI, 111, "Net is nullptr in DbNetDescriptor::highlight().");

[maliberty]

The singleton should only be used from the .i files not in the c++ core.

[maliberty]

All the descriptors assume they are given a non-null pointer. I prefer to fix this a level higher.