Intellicode#321 Fix invalid non array styleSheets item.

Author: Romick2005

lib/util/stylesheet.js

@@ -29,10 +29,10 @@ StyleSheets.prototype.markAsUsed = function (fullyQualifiedName) {
   const nameSplit = fullyQualifiedName.split('.');
   const styleSheetName = nameSplit[0];
   const styleSheetProperty = nameSplit[1];
+  const styleSheet = this.styleSheets[styleSheetName];
 
-  if (this.styleSheets[styleSheetName]) {
-    this.styleSheets[styleSheetName] = this
-      .styleSheets[styleSheetName]
+  if (Array.isArray(styleSheet)) {
+    this.styleSheets[styleSheetName] = styleSheet
       .filter((property) => property.key.name !== styleSheetProperty);
   }
 };