output: #fixed code
                               private void configWriter(String content, Path filePath) {
  		if(Files.exists(filePath) && !Files.isWritable(filePath)){
  			log.error("Error file is not writable: " + filePath);
  			return;
  		}
  		
  		if(Files.notExists(filePath.getParent())) {
  			try {
  				Files.createDirectories(filePath.getParent());
  			} catch (IOException e) {
  				log.error("Error creating the directory: " + filePath + " message: " + e.getMessage(), e);
  			}
  		}
  
  		try {
  			Path target = null;
  			if(Files.exists(filePath)) {
  				target = FileSystems.getDefault().getPath(filePath.getParent().toString(), "habridge.config.old");
  				Files.move(filePath, target);
  			}
  			Files.write(filePath, content.getBytes(), StandardOpenOption.CREATE);
  
  			// set attributes to be for user only
  	        // using PosixFilePermission to set file permissions
  	        Set<PosixFilePermission> perms = new HashSet<PosixFilePermission>();
  	        // add owners permission 
  	        perms.add(PosixFilePermission.OWNER_READ);
  	        perms.add(PosixFilePermission.OWNER_WRITE);
  	        
  	        try {
  	        	String osName = System.getProperty("os.name");
  	        	if(osName.toLowerCase().indexOf("win") < 0)
  	        		Files.setPosixFilePermissions(filePath, perms);
  	        } catch(UnsupportedOperationException e) {
  	        	log.info("Cannot set permissions for config file on this system as it is not supported. Continuing");
  	        }
  			if(target != null)
  				Files.delete(target);
  		} catch (IOException e) {
  			log.error("Error writing the file: " + filePath + " message: " + e.getMessage(), e);
  		}
  	}
input: #vulnerable code 
                               private void configWriter(String content, Path filePath) {
  		if(Files.exists(filePath) && !Files.isWritable(filePath)){
  			log.error("Error file is not writable: " + filePath);
  			return;
  		}
  		
  		if(Files.notExists(filePath.getParent())) {
  			try {
  				Files.createDirectories(filePath.getParent());
  			} catch (IOException e) {
  				log.error("Error creating the directory: " + filePath + " message: " + e.getMessage(), e);
  			}
  		}
  
  		try {
  			Path target = null;
  			if(Files.exists(filePath)) {
  				target = FileSystems.getDefault().getPath(filePath.getParent().toString(), "habridge.config.old");
  				Files.move(filePath, target);
  			}
  			Files.write(filePath, content.getBytes(), StandardOpenOption.CREATE);
  
  			// set attributes to be for user only
  	        // using PosixFilePermission to set file permissions
  	        Set<PosixFilePermission> perms = new HashSet<PosixFilePermission>();
  	        // add owners permission 
  	        perms.add(PosixFilePermission.OWNER_READ);
  	        perms.add(PosixFilePermission.OWNER_WRITE);
  	        
  	        try {
  	        	if(System.getProperty("os.name").toLowerCase().indexOf("win") <= 0)
  	        		Files.setPosixFilePermissions(filePath, perms);
  	        } catch(UnsupportedOperationException e) {
  	        	log.info("Cannot set permissions for config file on this system as it is not supported. Continuing");
  	        }
  			if(target != null)
  				Files.delete(target);
  		} catch (IOException e) {
  			log.error("Error writing the file: " + filePath + " message: " + e.getMessage(), e);
  		}
  	} 
                              #location 31 
                              #vulnerability type NULL_DEREFERENCE