Archive has to be cleared when clear() is called

src/core/qgsproject.cpp

@@ -494,6 +494,13 @@ void QgsProject::clear()
   mRootGroup->clear();
 
   mLabelingEngineSettings->clear();
+
+  // unzip action use read() method to read the embedded qgs project file.
+  // And read() clear everything in a first step. But we don't want to reset
+  // the current archive while unzipping...
+  if ( !mUnzipping )
+    mArchive->clear();
+
   emit labelingEngineSettingsChanged();
 
   // reset some default project properties
@@ -765,9 +772,6 @@ bool QgsProject::read()
 {
   clearError();
 
-  if ( ! mUnzipping )
-    mArchive->clear();
-
   std::unique_ptr<QDomDocument> doc( new QDomDocument( QStringLiteral( "qgis" ) ) );
 
   if ( !mFile.open( QIODevice::ReadOnly | QIODevice::Text ) )

tests/src/python/test_qgsproject.py

@@ -750,6 +750,9 @@ def test_zip_unzip(self):
         self.assertTrue(layers[l0.id()].isValid(), True)
         self.assertTrue(layers[l1.id()].isValid(), True)
 
+        project2.clear()
+        self.assertFalse(project2.unzipped())
+
 
 if __name__ == '__main__':
     unittest.main()