Add error check after getting clientset to make sure clientset is not nil

[Liujingfang1]

What this PR does / why we need it:
Current code doesn't have error check after getting clientset. If clientset is nil, the subsequent code will have runtime error. kubernetes/kubectl#226

Fixes kubernetes/kubectl#226
Special notes for your reviewer:

Release note:

NONE

[Liujingfang1]

/sig cli

[Liujingfang1]

/assign @mengqiy

[mengqiy]

/ok-to-test

[mengqiy]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Liujingfang1, mengqiy
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[mengqiy]

Can you update the PR description to include

fixes kubernetes/kubectl#226

[k8s-github-robot]

/test all

Tests are more than 96 hours old. Re-running tests.

[mengqiy]

@Liujingfang1 Please rebase :)

[k8s-ci-robot]

@Liujingfang1: PR needs rebase.

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[Liujingfang1]

The change has been covered by #60455