Fix crash when deleting a field in attribute table (fixes #12930)

Also fixes:
- multiple reloads of table (due to additional connections to cache invalidation)
- crash in attribute form (on redo of deleted field when it was not the last)
- crash in editor widget wrapper (during table reload)

Author: wonder-sk

src/gui/attributetable/qgsattributetablemodel.cpp

@@ -336,6 +336,13 @@ void QgsAttributeTableModel::loadAttributes()
 void QgsAttributeTableModel::loadLayer()
 {
   QgsDebugMsg( "entered." );
+
+  // make sure attributes are properly updated before caching the data
+  // (emit of progress() signal may enter event loop and thus attribute
+  // table view may be updated with inconsistent model which may assume
+  // wrong number of attributes)
+  loadAttributes();
+
   beginResetModel();
 
   if ( rowCount() != 0 )
@@ -370,9 +377,8 @@ void QgsAttributeTableModel::loadLayer()
 
   emit finished();
 
-  connect( mLayerCache, SIGNAL( invalidated() ), this, SLOT( loadLayer() ) );
+  connect( mLayerCache, SIGNAL( invalidated() ), this, SLOT( loadLayer() ), Qt::UniqueConnection );
 
-  mFieldCount = mAttributes.size();
   endResetModel();
 }
 

src/gui/editorwidgets/core/qgseditorwidgetwrapper.cpp

@@ -33,7 +33,10 @@ int QgsEditorWidgetWrapper::fieldIdx()
 
 QgsField QgsEditorWidgetWrapper::field()
 {
-  return layer()->pendingFields()[mFieldIdx];
+  if ( mFieldIdx < layer()->pendingFields().count() )
+    return layer()->pendingFields()[mFieldIdx];
+  else
+    return QgsField();
 }
 
 QVariant QgsEditorWidgetWrapper::defaultValue()

src/gui/qgsattributeform.cpp

@@ -277,8 +277,7 @@ void QgsAttributeForm::onAttributeAdded( int idx )
   if ( mFeature.isValid() )
   {
     QgsAttributes attrs = mFeature.attributes();
-    Q_ASSERT( attrs.size() == idx );
-    attrs.append( QVariant( layer()->pendingFields()[idx].type() ) );
+    attrs.insert( idx, QVariant( layer()->pendingFields()[idx].type() ) );
     mFeature.setFields( layer()->pendingFields() );
     mFeature.setAttributes( attrs );
   }