UnreachableBrowserException Caused by: java.lang.NullPointerException when“webdriver.firefox.marione

这是您问题的答案：

The error

UnreachableBrowserException: Could not start a new session.Possible causes are invalid address of the remote server or browser start-upfailure

As per guru99.com it is
mentioned to use

webdriver.firefox.marionette

System.setProperty

Selenium 3.x

webdriver.gecko.driver

So consider changing the line of pre:

System.setProperty("webdriver.firefox.marionette","C:\geckodriver.exe");

to:

System.setProperty("webdriver.gecko.driver","C:\your_directory\geckodriver.exe");

Let me know if this Answers your Question.