Fixes regressions in relation reference widget

src/gui/editorwidgets/qgsrelationreferencewidget.cpp

@@ -145,6 +145,7 @@ QgsRelationReferenceWidget::QgsRelationReferenceWidget( QWidget *parent )
   connect( mRemoveFKButton, &QAbstractButton::clicked, this, &QgsRelationReferenceWidget::deleteForeignKey );
   connect( mAddEntryButton, &QAbstractButton::clicked, this, &QgsRelationReferenceWidget::addEntry );
   connect( mComboBox, &QComboBox::editTextChanged, this, &QgsRelationReferenceWidget::updateAddEntryButton );
+  connect( mComboBox, &QgsFeatureListComboBox::modelUpdated, this, &QgsRelationReferenceWidget::updateIndex );
 }
 
 QgsRelationReferenceWidget::~QgsRelationReferenceWidget()
@@ -155,6 +156,38 @@ QgsRelationReferenceWidget::~QgsRelationReferenceWidget()
     delete mMapTool;
 }
 
+void QgsRelationReferenceWidget::updateIndex()
+{
+  if ( mChainFilters && mComboBox->count() > 0 )
+  {
+    int index = -1;
+
+    // uninitialized filter
+    if ( ! mFilterComboBoxes.isEmpty()
+         && mFilterComboBoxes[0]->currentIndex() == 0 && mAllowNull )
+    {
+      index = mComboBox->nullIndex();
+    }
+    else if ( mComboBox->count() > mComboBox->nullIndex() )
+    {
+      index = mComboBox->nullIndex() + 1;
+    }
+    else if ( mAllowNull )
+    {
+      index = mComboBox->nullIndex();
+    }
+    else
+    {
+      index = 0;
+    }
+
+    if ( mComboBox->count() > index )
+    {
+      mComboBox->setCurrentIndex( index );
+    }
+  }
+}
+
 void QgsRelationReferenceWidget::setRelation( const QgsRelation &relation, bool allowNullValue )
 {
   mAllowNull = allowNullValue;

src/gui/editorwidgets/qgsrelationreferencewidget.h

@@ -184,6 +184,12 @@ class GUI_EXPORT QgsRelationReferenceWidget : public QWidget
     void addEntry();
     void updateAddEntryButton();
 
+    /**
+     * Updates the FK index as soon as the underlying model is updated when
+     * the chainFilter option is activated.
+     */
+    void updateIndex();
+
   private:
     void highlightFeature( QgsFeature f = QgsFeature(), CanvasExtent canvasExtent = Fixed );
     void updateAttributeEditorFrame( const QgsFeature &feature );

src/gui/qgsfeaturelistcombobox.cpp

@@ -46,6 +46,7 @@ QgsFeatureListComboBox::QgsFeatureListComboBox( QWidget *parent )
   connect( mCompleter, static_cast<void( QCompleter::* )( const QModelIndex & )>( &QCompleter::activated ), this, &QgsFeatureListComboBox::onActivated );
   connect( mModel, &QgsFeatureFilterModel::beginUpdate, this, &QgsFeatureListComboBox::storeLineEditState );
   connect( mModel, &QgsFeatureFilterModel::endUpdate, this, &QgsFeatureListComboBox::restoreLineEditState );
+  connect( mModel, &QgsFeatureFilterModel::endUpdate, this, &QgsFeatureListComboBox::modelUpdated );
   connect( mModel, &QgsFeatureFilterModel::dataChanged, this, &QgsFeatureListComboBox::onDataChanged );
 
   connect( this, static_cast<void( QgsFeatureListComboBox::* )( int )>( &QgsFeatureListComboBox::currentIndexChanged ), this, &QgsFeatureListComboBox::onCurrentIndexChanged );
@@ -136,6 +137,18 @@ void QgsFeatureListComboBox::restoreLineEditState()
     mLineEditState.restore( mLineEdit );
 }
 
+int QgsFeatureListComboBox::nullIndex() const
+{
+  int index = -1;
+
+  if ( allowNull() )
+  {
+    index = findText( tr( "NULL" ) );
+  }
+
+  return index;
+}
+
 void QgsFeatureListComboBox::onDataChanged( const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles )
 {
   Q_UNUSED( roles )

src/gui/qgsfeaturelistcombobox.h

@@ -82,6 +82,14 @@ class GUI_EXPORT QgsFeatureListComboBox : public QComboBox
      */
     QString filterExpression() const;
 
+    /**
+     * Returns the current index of the NULL value, or -1 if NULL values are
+     * not allowed.
+     *
+     * \since QGIS 3.2
+     */
+    int nullIndex() const;
+
     /**
      * An additional expression to further restrict the available features.
      * This can be used to integrate additional spatial or other constraints.
@@ -141,6 +149,13 @@ class GUI_EXPORT QgsFeatureListComboBox : public QComboBox
 
   signals:
 
+    /**
+     * The underlying model has been updated.
+     *
+     * \since QGIS 3.2
+     */
+    void modelUpdated();
+
     /**
      * The layer from which features should be listed.
      */