What is the reason for the error message "lookup is null" when trying to invoke "com.vaadin.flow.di.Lookup.lookup(java.lang.Class)" in Vaadin?

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The error "lookup is null" occurs when there is no instance of the lookup class available in the Vaadin application or when the class that is being looked up is not found. This could happen if the dependency injection framework is not properly initialized or configured, or if the class being looked up is not registered with the framework. It is important to ensure that all dependencies are properly injected and registered with the lookup framework in order to avoid this error.