fix(module:table): set selectedRows exception

[qinhuaihe]

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<Modal Visible="false">
    <Table SelectedRows="SelectedRows">...</Table>
</Modal>

Setting SelectedRows before table rendered leads to a NullReferenceException:

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[ElderJames]

Thanks for contribution @zmayor .
We think the _dataSourceCache should be created instance before it is used.

[qinhuaihe]

Is this ok?

public partial class Table<TItem> : ITable
{
    private readonly IDictionary<int, RowData<TItem>> _dataSourceCache = new Dictionary<int, RowData<TItem>>();

    private void FlushCache()
    {
        _dataSourceCache.Clear();
    }
    ...
}

[anddrzejb]

@zmayor we already discussed the solution for this in #1138. I think your solution is ok, though @ElderJames suggested another one which may be more performant (a bit less allocation). I'd suggest to go with that one.