Fix: Fixed NPE in stream-healing

CityDoctorParent/Extensions/CityDoctorHealer/src/main/java/de/hft/stuttgart/citydoctor2/healer/Healer.java

@@ -13,6 +13,7 @@ import java.util.Map;
 import java.util.Objects;
 
 import de.hft.stuttgart.citydoctor2.database.CityObjectCache;
+import de.hft.stuttgart.citydoctor2.database.UnconnectedCache;
 import de.hft.stuttgart.citydoctor2.exceptions.CityObjectCacheException;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -189,7 +190,12 @@ public class Healer {
 				}
 			};
 		}
-		cache = checker.getModel().getCache();
+		if(checker.getModel() != null){
+			cache = checker.getModel().getCache();
+		} else {
+			//Stream healing is being used, use an unconnected cache
+			cache = new UnconnectedCache();
+		}
 		this.l = l;
 	}