Fix qapp fixture that was returning None.

Author: gqmelo

README.rst

@@ -111,6 +111,7 @@ Many thanks to:
 - Fabio Zadrozny (`@fabioz <https://github.com/fabioz>`_);
 - Datalyze Solutions (`@datalyze-solutions <https://github.com/datalyze-solutions>`_);
 - Florian Bruhin (`@The-Compiler <https://github.com/The-Compiler>`_);
+- Guilherme Quentel Melo (`@gqmelo <https://github.com/gqmelo>`_);
 
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pytestqt/plugin.py

@@ -525,7 +525,7 @@ def qapp():
     if app is None:
         global _qapp_instance
         _qapp_instance = QApplication([])
-        yield app
+        yield _qapp_instance
     else:
         yield app  # pragma: no cover