fix error while removing results that have no error (result is ok)

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/check/Checkable.java

@@ -224,7 +224,12 @@ public abstract class Checkable implements Serializable {
 	 * Clears the checkResults of this checkable unless the error occured during parsing.
 	 */
 	public void clearCheckResults() {
-		checkResults.entrySet().removeIf(e -> !e.getValue().getError().isParsingError());
+		checkResults.entrySet().removeIf(e -> {
+			if (e.getValue().getError() == null) {
+				return true;
+			}
+			return !e.getValue().getError().isParsingError();
+		});
 		setValidated(!checkResults.isEmpty());
 	}