QmlDesigner: Fix crash when example unavailable

Fix crash when opening an example that was there on startup, but got
removed during run time.

Pick-to: qds/4.7
Task-number: QDS-15305
Change-Id: Idc425768d1f141d26eddd71e934e99beb4680b02
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

share/qtcreator/qmldesigner/welcomepage/ThumbnailDelegate.qml

@@ -75,6 +75,7 @@ Item {
                 root.startDownload()
             } else if (downloadButton.alreadyDownloaded) {
                 root.clicked() // open example
+                extractor.probeTargetPath()
             }
         }
     }

src/plugins/qmldesigner/libs/qmldesignerutils/fileextractor.cpp

@@ -259,6 +259,11 @@ void FileExtractor::extract()
     m_unarchiver->start();
 }
 
+void FileExtractor::probeTargetPath()
+{
+    emit targetFolderExistsChanged();
+}
+
 void QmlDesigner::FileExtractor::removeTempTargetPath()
 {
     if (m_isTempTargetPath && m_targetPath.exists()) {

src/plugins/qmldesigner/libs/qmldesignerutils/fileextractor.h

@@ -59,6 +59,8 @@ public:
     Q_INVOKABLE void browse();
     Q_INVOKABLE void extract();
 
+    Q_INVOKABLE void probeTargetPath();
+
 signals:
     void targetPathChanged();
     void detailedTextChanged();

src/plugins/studiowelcome/studiowelcomeplugin.cpp

@@ -12,6 +12,7 @@
 #include <coreplugin/helpmanager.h>
 #include <coreplugin/icore.h>
 #include <coreplugin/imode.h>
+#include <coreplugin/messagebox.h>
 #include <coreplugin/modemanager.h>
 
 #include <projectexplorer/devicesupport/devicekitaspects.h>
@@ -290,14 +291,23 @@ public:
         QmlDesigner::QmlDesignerPlugin::emitUsageStatistics("exampleOpened:"
                                                             + exampleName);
 
-        const QString exampleFolder = examplePath + "/" + exampleName + "/";
+        const FilePath exampleFilePath = FilePath::fromString(examplePath) / exampleName;
 
-        QString projectFile = exampleFolder + exampleName + ".qmlproject";
+        FilePath projectFile = exampleFilePath / (exampleName + ".qmlproject");
 
         if (!explicitQmlproject.isEmpty())
-            projectFile = exampleFolder + explicitQmlproject;
+            projectFile = exampleFilePath / explicitQmlproject;
 
-        ProjectExplorer::ProjectExplorerPlugin::openProjectWelcomePage(FilePath::fromString(projectFile));
+        if (!projectFile.exists()) {
+            Core::AsynchronousMessageBox::warning(
+                tr("Could not open %1").arg(exampleName),
+                tr("Project file %1 is missing. Try downloading the example again.")
+                    .arg(projectFile.nativePath()));
+
+            return;
+        }
+
+        ProjectExplorer::ProjectExplorerPlugin::openProjectWelcomePage(projectFile);
     }
 
     Q_INVOKABLE void openExample(const QString &example,