Fix crash with right click on a pixel without color

Author: jbarandiaran

src/image_canvas.cpp

@@ -177,18 +177,19 @@ void ImageCanvas::mouseReleaseEvent(QMouseEvent * e) {
 	if (e->button() == Qt::RightButton) { // selection of label
 		QColor color = _mask.id.pixel(_mouse_pos / _scale);
 		const LabelInfo * label = _ui->id_labels[color.red()];
-
-		if (!_watershed.id.isNull() && _ui->checkbox_watershed_mask->isChecked()) {
-			QColor color = QColor(_watershed.id.pixel(_mouse_pos / _scale));
-			QMap<int, const LabelInfo*>::const_iterator it = _ui->id_labels.find(color.red());
-			if (it != _ui->id_labels.end()) {
-				label = it.value();
+		if (label != NULL)
+		{
+			if (!_watershed.id.isNull() && _ui->checkbox_watershed_mask->isChecked()) {
+				QColor color = QColor(_watershed.id.pixel(_mouse_pos / _scale));
+				QMap<int, const LabelInfo*>::const_iterator it = _ui->id_labels.find(color.red());
+				if (it != _ui->id_labels.end()) {
+					label = it.value();
+				}
 			}
+			if (label->item != NULL)
+				emit(_ui->list_label->currentItemChanged(label->item, NULL));
+			refresh();
 		}
-		if(label->item != NULL)
-			emit(_ui->list_label->currentItemChanged(label->item, NULL));
-
-		refresh();
 	}
 
 	if (e->button() == Qt::MiddleButton)