Android App Stop work ?

I just explained it all to you, and I cannot give you the code because I have no idea where and when you are calling the function. You did not explain it well, where is the code? The exception only does not help in answering the question. 

 

The rest is up to you, you have to solve it. Even if I solve this one, what about the next one? Would you ask someone to solve that for you? :-) You should try to solve it yourself.  

The objects that have not yet initialized using a value are null. You can have a look at all of those types of objects that do not have a value. They are null and will give a problem. For example in Java, 

String nullStr;
String initializedStr = "";

In the above code the nullStr is null (because it does not have a value), but on the other hand initializedStr does have a value, an empty string. You should read this blog of mine to learn more about this, https://basicsofwebdevelopment.wordpress.com/2014/07/03/what-is-a-null-error-in-code-execution/

 

In your code, the Toolbar is null. Have a look at your code,

Attempt to invoke virtual method 'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()' on a null object reference

This tells you th exact point where the problem occurs.  Make sure that Toolbar is initialized, or do not call this function on it. 

 

Finally, you do not remove null error. You fix it, by initializing the object. :-)