Prevent creating invalid join in add join dialog (fixes crash)

(cherry-picked from 693ead2)

Author: nyalldawson

src/app/qgsjoindialog.cpp

@@ -24,6 +24,7 @@
 #include "qgsfieldcombobox.h"
 
 #include <QStandardItemModel>
+#include <QPushButton>
 
 QgsJoinDialog::QgsJoinDialog( QgsVectorLayer* layer, QList<QgsMapLayer*> alreadyJoinedLayers, QWidget * parent, Qt::WindowFlags f )
     : QDialog( parent, f )
@@ -53,6 +54,12 @@ QgsJoinDialog::QgsJoinDialog( QgsVectorLayer* layer, QList<QgsMapLayer*> already
     mJoinFieldComboBox->setLayer( joinLayer );
     joinedLayerChanged( joinLayer );
   }
+
+  connect( mJoinLayerComboBox, SIGNAL( layerChanged( QgsMapLayer* ) ), this, SLOT( checkDefinitionValid() ) );
+  connect( mJoinFieldComboBox, SIGNAL( fieldChanged( QString ) ), this, SLOT( checkDefinitionValid() ) );
+  connect( mTargetFieldComboBox, SIGNAL( fieldChanged( QString ) ), this, SLOT( checkDefinitionValid() ) );
+
+  checkDefinitionValid();
 }
 
 QgsJoinDialog::~QgsJoinDialog()
@@ -98,7 +105,8 @@ void QgsJoinDialog::setJoinInfo( const QgsVectorJoinInfo& joinInfo )
 QgsVectorJoinInfo QgsJoinDialog::joinInfo() const
 {
   QgsVectorJoinInfo info;
-  info.joinLayerId = mJoinLayerComboBox->currentLayer()->id();
+  if ( mJoinLayerComboBox->currentLayer() )
+    info.joinLayerId = mJoinLayerComboBox->currentLayer()->id();
   info.joinFieldName = mJoinFieldComboBox->currentField();
   info.targetFieldName = mTargetFieldComboBox->currentField();
   info.memoryCache = mCacheInMemoryCheckBox->isChecked();
@@ -173,3 +181,10 @@ void QgsJoinDialog::joinedLayerChanged( QgsMapLayer* layer )
     mCustomPrefix->setText( layer->name() + '_' );
   }
 }
+
+void QgsJoinDialog::checkDefinitionValid()
+{
+  buttonBox->button( QDialogButtonBox::Ok )->setEnabled( mJoinLayerComboBox->currentIndex() != -1
+      && mJoinFieldComboBox->currentIndex() != -1
+      && mTargetFieldComboBox->currentIndex() != -1 );
+}

src/app/qgsjoindialog.h

@@ -42,6 +42,8 @@ class APP_EXPORT QgsJoinDialog: public QDialog, private Ui::QgsJoinDialogBase
   private slots:
     void joinedLayerChanged( QgsMapLayer* layer );
 
+    void checkDefinitionValid();
+
   private:
     /** Target layer*/
     QgsVectorLayer* mLayer;