[#6265] NullReferenceException when passing dialog name as variable to Microsoft.BeginDialog

[sw-joelmut]

Fixes #6265
#minor

Description

This PR adds the ability for the Dialog to be loaded automatically from an expression property when isn't registered manually.
Once the dialog is loaded into the dialogs stack, the next time the dialog is required, it will be already available.

Specific Changes

• Adds a step to gather the dialog resource when isn't found in the dialogs stack from the ResourceExplorer to be loaded automatically into the dialogs stack.
• Adds a new unit test that corroborates the dialog is being loaded correctly from an expression property.

Testing

The following image shows the dialog working from a Composer bot and the new unit test passing.

[coveralls]

Pull Request Test Coverage Report for Build 307917

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.02%) to 78.969%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs.Adaptive/Actions/BaseInvokeDialog.cs	1	93.33%
/libraries/Microsoft.Bot.Builder.Dialogs.Adaptive/Recognizers/CrossTrainedRecognizerSet.cs	1	95.77%
/libraries/Microsoft.Bot.Connector.Streaming/Session/StreamingSession.cs	3	89.62%
/libraries/Microsoft.Bot.Builder.Dialogs/Recognizer.cs	12	66.67%

Totals
Change from base Build 304888:	0.02%
Covered Lines:	25391
Relevant Lines:	32153

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💛 - Coveralls

[gabog]

Hi @EricDahlvang, can you take a look at this PR? It looks good to me but would like to get a second opinion.

Thanks

[EricDahlvang]

Thanks @sw-joelmut !