Only modify source map if it's not null

Null checks are why we can't have nice things.

src/React.Core/Babel.cs

@@ -232,12 +232,15 @@ protected virtual JavaScriptWithSourceMap TransformWithHeader(
 			result.Code = header + result.Code;
 			result.Hash = hash;
 
-			// Since we prepend a header to the code, the source map no longer matches up exactly
-			// (it's off by the number of lines in the header). Fix this issue by adding five 
-			// blank lines to the source map. This is kinda hacky but saves us having to load a
-			// proper source map library. If this ever breaks, I'll replace it with actual proper
-			// source map modification code (https://gist.github.com/Daniel15/4bdb15836bfd960c2956).
-			result.SourceMap.Mappings = ";;;;;" + result.SourceMap.Mappings;
+			if (result.SourceMap != null && result.SourceMap.Mappings != null)
+			{
+				// Since we prepend a header to the code, the source map no longer matches up exactly
+				// (it's off by the number of lines in the header). Fix this issue by adding five 
+				// blank lines to the source map. This is kinda hacky but saves us having to load a
+				// proper source map library. If this ever breaks, I'll replace it with actual proper
+				// source map modification code (https://gist.github.com/Daniel15/4bdb15836bfd960c2956).
+				result.SourceMap.Mappings = ";;;;;" + result.SourceMap.Mappings;
+			}
 
 			return result;
 		}