Fix crash when adding Table/Chart/Matrix to a report with no datasets

[Copilot]

Opening a Table, Chart, or Matrix dialog on an empty report (no datasets defined) throws an unhandled ArgumentException because cbDataSets.SelectedIndex = 0 is called unconditionally on an empty ComboBox.

Changes

• DialogNewTable.cs, DialogNewChart.cs, DialogNewMatrix.cs: Guard SelectedIndex assignment with an item count check in each dialog's constructor.

// Before — throws when no datasets exist
cbDataSets.SelectedIndex = 0;

// After — safe when ComboBox is empty
if (cbDataSets.Items.Count > 0)
    cbDataSets.SelectedIndex = 0;

Original prompt

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This section details on the original issue you should resolve

<issue_title>InvalidArgument=Value of '0'</issue_title>
<issue_description>Describe the bug
A clear and concise description of what the bug is.
Designer is getting errors
To Reproduce
Steps to reproduce the behavior:

1. Go to 'Firstly create an empty report"
2. Click Tables/Chart/List'
3. Click on ''body"
4. See error

Expected behavior
Create a table

Screenshots

Additional context
Add any other context about the problem here.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes InvalidArgument=Value of '0' #264

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[Copilot]

Pull request overview

This pull request fixes a crash that occurs when opening Table, Chart, or Matrix dialogs on reports with no datasets defined. The issue is caused by unconditionally setting SelectedIndex = 0 on an empty ComboBox, which throws an ArgumentException.

Changes:

• Added defensive guards to check Items.Count > 0 before setting SelectedIndex = 0 in three dialog initialization methods
• The fix prevents the crash while maintaining correct behavior when datasets are present
• The pattern used matches existing conventions in the codebase (e.g., DialogDataSources, ReportParameterCtl)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
RdlDesign/DialogNewTable.cs	Added guard to prevent setting SelectedIndex on empty cbDataSets ComboBox
RdlDesign/DialogNewMatrix.cs	Added guard to prevent setting SelectedIndex on empty cbDataSets ComboBox
RdlDesign/DialogNewChart.cs	Added guard to prevent setting SelectedIndex on empty cbDataSets ComboBox

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