Issue: Object reference not set error when open Sitecore Path Analyzer After upgrade from 7.5 to 8.1 update 2
How ever this error was present in 8.1 Initial update too. Thanks to chris who give me direction where to look it save my time. However that solution is not fix actual problem it It show sitecore root in select map dropdown which is not correct list of maps.
When I try to open Path Analyzer 
for the first time after upgrade to 8.1 update 2 and Initial release, I get an follwong error in the MapSelector.cshtml file, Object 
reference not set to an instance of an object.
helper.MakeTreeDefinition("TreeDefinitionFilter", 
fieldsWrapper, selectedTreeDefinitionId.ToString(), mapRootId.ToString());
The mapRootId object.  That parameter is empty when launching the 
Path Analyser, so the method expecting this value but  finds nothing.  This 
happens on line 32: 
var mapRootId = 
parametersHelper.GetMapRootId();
The  problem is that GetMapRootId() is retuning a null value from this command, when 
that Mapselector Component parameter doesn’t exist.  
Solutions:
To fix above issue follow the steps below
1. Go Sitecore Desktop
2. switch Database to Core
3. Go  to the path below
 /sitecore/client/Applications/PathAnalyzer/Common/Layouts/Renderings/MapSelector/MapSelector Parameters/__Standard Values
4.Find Field called DefaultMapRoot  set following id {6768BE32-A142-4BA4-A02F-01D810FBF7FE}
5. close  sitecore clean browser cookies
6. Go to Path Analyzer .................
 
Enjoy