Fix NPE in ManagedInformerEventSource #6963

Job	Run time
sample_operators_tests (sample-operators/operations)	12m 1s
sample_operators_tests (sample-operators/webpage)	3m 10s
sample_operators_tests (sample-operators/leader-election)	3m 17s
sample_operators_tests (sample-operators/tomcat-operator)	3m 1s
sample_operators_tests (sample-operators/mysql-schema)	2m 36s
sample_operators_tests (sample-operators/operations)	8m 27s
sample_operators_tests (sample-operators/webpage)	3m 10s
sample_operators_tests (sample-operators/tomcat-operator)	3m 1s
sample_operators_tests (sample-operators/leader-election)	3m 17s
sample_operators_tests (sample-operators/mysql-schema)	2m 36s
	44m 36s

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