fix broken project after deletion

src/main/java/be/uantwerpen/mime_webapp/model/Project.java

@@ -49,7 +49,15 @@ public class Project implements Serializable{
 	{
 		this.items.remove(item);
 		File file = item.getFile();
+
 		if(file.exists()){
+			//we don't remove if the file is still used by other items
+			for(FileItem fileitem : items) {
+				if(fileitem.getFile().equals(file)) {
+					return;
+				}
+			}
+
 			file.delete();
 			System.err.println("Deleting file " + file.getAbsolutePath());
 		}