Fix Editor crash in test teardown calling set_view_pane_layout

If Atom isn't initialized and able to produce a ViewportContext, the Editor would crash. This attempts to make the initialization fail a bit more gracefully and fixes the crash in the cases I've tested.

Gems/Atom/Tools/MaterialEditor/Code/Source/Viewport/MaterialViewportWidget.cpp

@@ -38,7 +38,7 @@ namespace MaterialEditor
 {
 
     MaterialViewportWidget::MaterialViewportWidget(QWidget* parent)
-        : AtomToolsFramework::RenderViewportWidget(AzFramework::InvalidViewportId, parent)
+        : AtomToolsFramework::RenderViewportWidget(parent)
         , m_ui(new Ui::MaterialViewportWidget)
     {
         m_ui->setupUi(this);