Word Export: writing system that is not part of the project

Prevent Word Export from crashing when we encounter a string
that uses a writing system that is not part of the current
FLEX project. This can happen when using copy/paste from one
FLEX project to another, and the projects use different
writing systems. See LT-21981.
Pasting table data from a custom field in one project to a
custom field in another project reproduced the crash.

Change-Id: Ic238b27a83e9591bc81eac9bdf4628ab11787352

Src/xWorks/LcmWordGenerator.cs

@@ -749,7 +749,8 @@ public void AddRun(LcmCache cache, ConfigurableDictionaryNode config, ReadOnlyPr
 							else
 							{
 								StyleElement rootElem = s_styleCollection.GetStyleElement(wsString);
-								Style rootStyle = rootElem.Style;
+								// rootElem can be null, see LT-21981.
+								Style rootStyle = rootElem?.Style;
 								if (rootStyle != null)
 								{
 									Style basedOnStyle = WordStylesGenerator.GenerateBasedOnCharacterStyle(new Style(), wsString, displayNameBase);