What does the groovy.lang.MissingPropertyException message mean when it states that there is no property called 'app' for the javaposse.jobdsl.dsl.helpers.workflow.CpsContext class?

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The groovy.lang.MissingPropertyException message means that there is an attempt to access a property called 'app' for the javaposse.jobdsl.dsl.helpers.workflow.CpsContext class, but this property does not exist. This can be due to a typo in the property name or to the fact that the property has not been defined or initialized.