Fix possible crash in QmlJS semantic checker.

src/libs/qmljs/qmljscheck.cpp

@@ -190,8 +190,11 @@ void Check::checkScopeObjectMember(const AST::UiQualifiedId *id)
         return;
 
     // attached properties
-    if (! propertyName.isEmpty() && propertyName[0].isUpper())
+    if (! propertyName.isEmpty() && propertyName[0].isUpper()) {
         scopeObject = _context.typeEnvironment(_doc.data());
+        if (! scopeObject)
+            return;
+    }
 
     const Value *value = scopeObject->lookupMember(propertyName, &_context);
     if (_extraScope && !value)