Fix: Add non-null check for file

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/CityDoctorModel.java

@@ -75,6 +75,9 @@ public class CityDoctorModel {
         if (config == null) {
             throw new IllegalArgumentException("Parser configuration may not be null");
         }
+        if( file == null ) {
+            throw new IllegalArgumentException("File may not be null");
+        }
         this.fileName = file.getName();
         this.config = config;
         this.file = file;