[Canvas] Avoid exception when no embedded view is available

Partially addresses nasa/mct#220

* Avoid assuming that embedded (canvas-eligible) views are available (user plug-ins may introduce components which support no embedded views) in code
* When no embedded view is available, show an "empty" Panel

```
 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Self-review by pull request
 Describe potential impact on other areas of the code: Canvas
 Describe any test prerequisites for this fix (e.g., restart the database): Run with plug-in which has a component with no embedded view (e.g. Activity Type in Scenario plug-in)
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N
```