playground: add panic guard to prevent some corner cases

[xhebox]

What problem does this PR solve?

When something is wrong, and tiup did not start the component correctly, playground instance may have a nil process pointer. Let us just prevent it from panic.

Refer https://do.pingcap.net/jenkins/blue/organizations/jenkins/tikv%2Fpd%2Fpull_integration_realcluster_test/detail/pull_integration_realcluster_test/809/pipeline/.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

Prevent possible panic on corner cases of playground startup failure

[ti-chi-bot]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-05 03:20:43.641799055 +0000 UTC m=+500968.159851980: ☑️ agreed by breezewish.