[OffspecDetectorEditor] Fix layout warning in OffspecDetectorEditor ()

Merging branch 'OffspecDetectorEditor'  into 'main'.

See merge request !1441

GUI/View/Instrument/OffspecDetectorEditor.cpp

@@ -37,7 +37,7 @@ OffspecDetectorEditor::OffspecDetectorEditor(QWidget* parent, OffspecInstrumentI
 
     OffspecDetectorItem* detector_item = m_instrument->detectorItem();
 
-    auto* grid = new QGridLayout(this);
+    auto* grid = new QGridLayout;
     grid->setColumnStretch(0, 1);
     grid->setColumnStretch(1, 1);
     grid->setColumnStretch(2, 1);