Guard delayed document initialization against empty paragraph state

[tobitege]

Summary

• Query the current paragraph collection defensively when the delayed initialization callback runs.
• Skip focus and text-layout initialization when the document has been cleared or replaced before that callback executes.

Root cause

InitializeDocument() posts work at DispatcherPriority.Background. The callback previously accessed the first paragraph with [0], relying on state that was valid when the work was scheduled. A host can clear or replace FlowDocument.Blocks before the dispatcher executes the callback, leaving the paragraph collection empty and causing an ArgumentOutOfRangeException from the dispatcher.

Fix

Use FirstOrDefault() inside the delayed callback and run the focus/layout initialization only when a paragraph still exists. If the document state has already been replaced, the obsolete initialization work becomes a no-op.

Validation

• dotnet build AvRichTextBox.sln completed with 0 errors.
• Launched DemoApp_AvRichTextBox. Its loaded path clears and replaces FlowDocument.Blocks after control initialization; the application remained responsive and rendered the replacement paragraph and table.