QmlDesigner: Improve error message on trying to edit empty component

Task-number: QDS-5308
Change-Id: I401a375443e2df52a2656a34ab4c650664b0df97
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

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

@@ -123,6 +123,9 @@ QTextDocument *ComponentTextModifier::textDocument() const
 
 QString ComponentTextModifier::text() const
 {
+    if (m_componentStartOffset == -1)
+        return {};
+
     QString txt(m_originalModifier->text());
 
     const int leader = m_componentStartOffset - m_rootStartOffset;

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

@@ -1044,6 +1044,15 @@ void TextToModelMerger::setupUsedImports()
 
 Document::MutablePtr TextToModelMerger::createParsedDocument(const QUrl &url, const QString &data, QList<DocumentMessage> *errors)
 {
+    if (data.isEmpty()) {
+        if (errors) {
+            QmlJS::DiagnosticMessage msg;
+            msg.message = QObject::tr("Empty document");
+            errors->append(DocumentMessage(msg, url));
+        }
+        return {};
+    }
+
     const QString fileName = url.toLocalFile();
 
     Dialect dialect = ModelManagerInterface::guessLanguageOfFile(fileName);