Fix crash when exporting SQL files

Exporting SQL files crashed because of the multi-threaded loading of
data which also affects the SQL export. Because the table models were
destructed relatively quickly during the export's progress estimation,
their threads were still running and trying to access invalid data
structures.

See issue #1228.

src/sqlitetablemodel.cpp

@@ -22,6 +22,12 @@ SqliteTableModel::SqliteTableModel(DBBrowserDB& db, QObject* parent, size_t chun
     reset();
 }
 
+SqliteTableModel::~SqliteTableModel()
+{
+    m_futureFetch.cancel();
+    m_futureFetch.waitForFinished();
+}
+
 void SqliteTableModel::reset()
 {
     m_futureFetch.cancel();

src/sqlitetablemodel.h

@@ -20,6 +20,7 @@ class SqliteTableModel : public QAbstractTableModel
 
 public:
     explicit SqliteTableModel(DBBrowserDB& db, QObject *parent = nullptr, size_t chunkSize = 50000, const QString& encoding = QString());
+    ~SqliteTableModel();
     void reset();
 
     int rowCount(const QModelIndex &parent = QModelIndex()) const;