[WASM][TokenizingTextBox] TokenizingTextBox suggestion don't render properly

[marcelwgn]

Describe the bug

A clear and concise description of what the bug is.

TokenizingTextBox in WASM does not work:

• If the SelectionItemsSource is not changed, no dropdown is being rendered

• If you change the SelectionItemsSource and select an item, multiple dropdowns are visible on screen

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? (Either in a sample as-is or with new XAML pasted in the editor.) If so, please provide custom XAML or steps to reproduce. If not, let us know why it can't be reproduced (e.g. more complex setup, environment, dependencies, etc...)

Steps to reproduce the behavior:
See attached repro project:

App7.zip

Expected behavior

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Screenshots

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Environment

NuGet Package(s): 

Package Version(s): 

Device form factor:
- [ ] Windows
- [ ] macOS
- [ ] iOS
- [ ] Android
- [x] WebAssembly
- [ ] WebAssembly renderers for Xamarin.Forms

Visual Studio 
- [ ] 2017 (version: )
- [x] 2019 (version: 16.10.0) 
- [ ] 2019 Preview (version: )

Additional context

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