Check whether propertiesFile is null before accessing (#1647)

Travis looks like it's stalled. I'm going to force this in so we can build an insiders package.

Author: grdowns

Extension/src/LanguageServer/configurations.ts

@@ -202,7 +202,7 @@ export class CppProperties {
             config.cppStandard = this.defaultCppStandard;
             doUpdate = true;
         }
-        if (doUpdate) {
+        if (doUpdate && this.propertiesFile) {
             fs.writeFileSync(this.propertiesFile.fsPath, JSON.stringify(this.configurationJson, null, 4));
         }