[REEF-1646] Adding ArgumentNullException in config initialization

JIRA: [REEF-1646](https://issues.apache.org/jira/browse/REEF-1646)
This closes #1173
diff --git a/lang/cs/Org.Apache.REEF.Tang/Implementations/Configuration/ConfigurationBuilderImpl.cs b/lang/cs/Org.Apache.REEF.Tang/Implementations/Configuration/ConfigurationBuilderImpl.cs
index fe3a099..4eab9a1 100644
--- a/lang/cs/Org.Apache.REEF.Tang/Implementations/Configuration/ConfigurationBuilderImpl.cs
+++ b/lang/cs/Org.Apache.REEF.Tang/Implementations/Configuration/ConfigurationBuilderImpl.cs
@@ -59,6 +59,11 @@
             this.ClassHierarchy = TangFactory.GetTang().GetDefaultClassHierarchy(assemblies, parsers);
             foreach (IConfiguration tc in confs) 
             {
+                if (tc == null)
+                {
+                    throw new ArgumentNullException("One of specified configurations is null");
+                } 
+                
                 AddConfiguration((ConfigurationImpl)tc);
             }
         }