[Formatting] Added null check.

ICSharpCode.NRefactory.CSharp/Formatter/AstFormattingVisitor.cs

@@ -863,11 +863,13 @@ namespace ICSharpCode.NRefactory.CSharp
 				}
 			} else {
 				foreach (var arg in parameters) {
+					if (arg.PrevSibling != null) {
 						if (methodCallArgumentWrapping == Wrapping.DoNotWrap) {
 							ForceSpacesBeforeRemoveNewLines(arg, spaceAfterMethodCallParameterComma && arg.PrevSibling.Role == Roles.Comma);
 						} else {
 							ForceSpacesBefore(arg, spaceAfterMethodCallParameterComma && arg.PrevSibling.Role == Roles.Comma);
 						}
+					}
 					arg.AcceptVisitor(this);
 				}
 				ForceSpacesBeforeRemoveNewLines(rParToken, spaceWithinMethodCallParentheses);
@@ -1764,11 +1766,13 @@ namespace ICSharpCode.NRefactory.CSharp
 				}
 			} else {
 				foreach (var arg in arguments) {
+					if (arg.PrevSibling != null) {
 						if (methodCallArgumentWrapping == Wrapping.DoNotWrap) {
 							ForceSpacesBeforeRemoveNewLines(arg, spaceAfterMethodCallParameterComma && arg.PrevSibling.Role == Roles.Comma);
 						} else {
 							ForceSpacesBefore(arg, spaceAfterMethodCallParameterComma && arg.PrevSibling.Role == Roles.Comma);
 						}
+					}
 					arg.AcceptVisitor(this);
 				}
 				ForceSpacesBeforeRemoveNewLines(rParToken, spaceWithinMethodCallParentheses);