Crash when running query with columns selected for plot

The model was still changing when the plot was being drawn. Now the plot
waits for the fetching to be finished.

See issue #1262

src/PlotDock.cpp

@@ -85,6 +85,8 @@ void PlotDock::updatePlot(SqliteTableModel* model, BrowseDataTableSettings* sett
 
         if(model)
         {
+            model->waitForFetchingFinished();
+
             // Add each column with a supported data type to the column selection view
             for(int i=0;i<model->columnCount();++i)
             {