panic in the TestColumnAdd

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x90 pc=0x73c51f6]
goroutine 39441 [running]:
github.com/pingcap/tidb/pkg/ddl_test.checkAddPublic({0x9d50a08, 0xc00881bef0}, {0x0, 0x0}, {0x9d0ce28?, 0xc007461ce0?})
	pkg/ddl/column_change_test.go:316 +0x1d6
github.com/pingcap/tidb/pkg/ddl_test.TestColumnAdd.func1(0xc00851fb00)
	pkg/ddl/column_change_test.go:80 +0x286
github.com/pingcap/tidb/pkg/ddl/util/callback.(*TestDDLCallback).OnJobUpdated(0xc0087789a0, 0xc00851fb00?)
	pkg/ddl/util/callback/callback.go:124 +0x1dd
github.com/pingcap/tidb/pkg/ddl.(*ddl).delivery2worker.func1()
	pkg/ddl/job_table.go:439 +0x171b
github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run.func1()
	pkg/util/wait_group_wrapper.go:157 +0x87
created by github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run in goroutine 38134
	pkg/util/wait_group_wrapper.go:155 +0xe5 

4. What is your TiDB version? (Required)

[hawkingrei]

https://tiprow.hawkingrei.com/view/gs/pingcapprow/logs/tidb_data_race/1739835333379362816#1:build-log.txt%3A6979