[bug][minor] Non-string value of `sorting-spec:` prevents the custom sorting from working w/o error message

[SebastianMC]

When the value of sorting-spec: is entered - by mistake - as an object, not as a string, the plugin stops from working. No error message is shown. Any attempt to enable custom sorting (ribbon icon, command, context menu) fails with no error message. Only JS exception is visible in developer console.

For example, missing the trailing pipe character | gives this effect:

sorting-spec:
  target-folder: /*
  /: ... \-D+

For clarity, the correct version of the spec is:

sorting-spec: |
  target-folder: /*
  /: ... \-D+

or

sorting-spec: |-
  target-folder: /*
  /: ... \-D+

The latter form with |- is generated by the Obsidian properties editor.

[SebastianMC]

Fix committed to master branch, will be included in next release. The issue seems to be not deserving a new release.