requesthandler: Fix AcquireSource without canvas

[exeldro]

Description

Fix AcquireSource without canvas

Motivation and Context

Don't like broken stuff
Fixing #1333

How Has This Been Tested?

Tested OS(s): Windows 11

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.

[tt2468]

Approving this as it does fix the issue. Unfortunately, the error messages will be a bit awkward for inputs, since they suggest that an input was not found in some canvas, even though inputs cannot exist in a canvas at all. I think the oversight is minor enough to not be worth delaying this fix, though.