Fix NullReferenceException in StairstepSeries (#605)

CHANGELOG.md

@@ -98,6 +98,7 @@ All notable changes to this project will be documented in this file.
 - Fix rendering of rotated 'math' text (#569, #448)
 - Export demo (WPF) (#568)
 - Fixing a double comparison issue causing infinite loop (#587)
+- Fix null reference exception when ActualPoints was null rendering a StairStepSeries (#582) 
 
 ## [2014.1.546] - 2014-10-22
 ### Added

CONTRIBUTORS

@@ -19,6 +19,7 @@ brantheman
 Brannon King
 Brian Lim <brian.lim.ca@gmail.com>
 Caleb Clarke <thealmightybob@users.noreply.github.com>
+Carlos Teixeira <karlmtc@gmail.com>
 Cyril Martin <cyril.martin.cm@gmail.com>
 darrelbrown
 David Laundav <davelaundav@gmail.com>