The interface ProcessApplicationElResolver can be implemented to resolve Expression Language objects from ProcessApplication-specific resources.
E.g. the engine-cdi and engine-spring module provide such a resolver.
- the engine throws a NullPointerException when a resolver implementation returns null. This can be the case when the engine-spring module is in the classpath but not used by a ProcessApplication
- the engine ignores null resolvers and logs a warning that no resolver could be retrieved from a ProcessApplicationElResolver implementation that was found on the classpath
- add a null check to DefaultElResolverLookup#lookupResolver(..)