Fix NPE in FolderObserver (#4896)

Fix #4890

Signed-off-by: Laurent Garnier <lg.hc@free.fr>

Author: lolodomo

bundles/org.openhab.core.model.core/src/main/java/org/openhab/core/model/core/internal/folder/FolderObserver.java

@@ -24,6 +24,7 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
@@ -113,7 +114,8 @@ protected void removeModelParser(ModelParser modelParser) {
         parsers.remove(extension);
 
         Set<String> removed = modelRepository.removeAllModelsOfType(extension);
-        ignoredPaths.addAll(removed.stream().map(namePathMap::get).collect(Collectors.toSet()));
+        ignoredPaths
+                .addAll(removed.stream().map(namePathMap::get).filter(Objects::nonNull).collect(Collectors.toSet()));
     }
 
     @Activate