Guard against null sceneGraphContext() in QQuickShaderEffect shutdown

This fixes a shutdown crash observed in Qt Design Studio 4.8 with Qt 6.8.3.

Minimal crash trace:
  QQuickShaderEffectPrivate::shaderEffectManager
  QQuickShaderEffectPrivate::updateShader
  QQuickShaderEffect::setVertexShader
  ...
  QQmlBinding::slowWrite
  QObject::~QObject

Pick-to: 6.9 6.8
Change-Id: I0e729bf7bb59a4e133cc2d06d79af2343934bbc5
Reviewed-by: Ulf Hermann <[email protected]>
Reviewed-by: Tor Arne Vestbø <[email protected]>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/quick/items/qquickshadereffect.cpp b/src/quick/items/qquickshadereffect.cpp
index c956977301..2a0659c250 100644
--- a/src/quick/items/qquickshadereffect.cpp
+++ b/src/quick/items/qquickshadereffect.cpp
@@ -1181,7 +1181,9 @@ QSGGuiThreadShaderEffectManager *QQuickShaderEffectPrivate::shaderEffectManager(
             return m_mgr;
         QQuickWindow *w = q->window();
         if (w) { // note: just the window, don't care about isSceneGraphInitialized() here
-            m_mgr = QQuickWindowPrivate::get(w)->context->sceneGraphContext()->createGuiThreadShaderEffectManager();
+            QSGRenderContext *renderContext = QQuickWindowPrivate::get(w)->context;
+            if (QSGContext *sgContext = renderContext->sceneGraphContext())
+                m_mgr = sgContext->createGuiThreadShaderEffectManager();
             if (m_mgr) {
                 QObject::connect(m_mgr, &QSGGuiThreadShaderEffectManager::logAndStatusChanged, q, &QQuickShaderEffect::logChanged);
                 QObject::connect(m_mgr, &QSGGuiThreadShaderEffectManager::logAndStatusChanged, q, &QQuickShaderEffect::statusChanged);