Model.Rewriter: crash fix

No TextModifer might be attached.

Change-Id: Ia06a1844ebf10392de1edb8cdb25d70bfa31e14d
Reviewed-by: Marco Bubke <marco.bubke@digia.com>

src/plugins/qmldesigner/designercore/model/rewriterview.cpp

@@ -486,12 +486,14 @@ QString RewriterView::textModifierContent() const
 
 void RewriterView::reactivateTextMofifierChangeSignals()
 {
-    textModifier()->reactivateChangeSignals();
+    if (textModifier())
+        textModifier()->reactivateChangeSignals();
 }
 
 void RewriterView::deactivateTextMofifierChangeSignals()
 {
-    textModifier()->deactivateChangeSignals();
+    if (textModifier())
+        textModifier()->deactivateChangeSignals();
 }
 
 void RewriterView::applyModificationGroupChanges()