Gui: Improve document recovery

When checking for recovery files also check for validity of the
actual project file. In case it's broken but has a never date
then still process the recovery file.

This is done to reduce the chance of data loss as described in
issue 18044

src/Gui/DocumentRecovery.cpp

@@ -49,6 +49,7 @@
 
 #include <App/Application.h>
 #include <App/Document.h>
+#include <App/ProjectFile.h>
 #include <Base/Exception.h>
 #include <Gui/Application.h>
 #include <Gui/Command.h>
@@ -164,6 +165,7 @@ public:
     QList<Info> recoveryInfo;
 
     Info getRecoveryInfo(const QFileInfo&) const;
+    bool isValidProject(const QFileInfo&) const;
     void writeRecoveryInfo(const Info&) const;
     XmlConfig readXmlFile(const QString& fn) const;
 };
@@ -433,7 +435,7 @@ DocumentRecoveryPrivate::Info DocumentRecoveryPrivate::getRecoveryInfo(const QFi
         if (info.status == DocumentRecoveryPrivate::Created) {
             // compare the modification dates
             QFileInfo fileInfo(info.fileName);
-            if (!info.fileName.isEmpty() && fileInfo.exists()) {
+            if (!info.fileName.isEmpty() && isValidProject(fileInfo)) {
                 QDateTime dateRecv = QFileInfo(file).lastModified();
                 QDateTime dateProj = fileInfo.lastModified();
                 if (dateRecv < dateProj) {
@@ -449,6 +451,16 @@ DocumentRecoveryPrivate::Info DocumentRecoveryPrivate::getRecoveryInfo(const QFi
     return info;
 }
 
+bool DocumentRecoveryPrivate::isValidProject(const QFileInfo& fi) const
+{
+    if (!fi.exists()) {
+        return false;
+    }
+
+    App::ProjectFile project(fi.absoluteFilePath().toStdString());
+    return project.loadDocument();
+}
+
 DocumentRecoveryPrivate::XmlConfig DocumentRecoveryPrivate::readXmlFile(const QString& fn) const
 {
     DocumentRecoveryPrivate::XmlConfig cfg;