Fix Model error messages when first created

[LogicalError]

When you create a model, before any brushes are added, it'll show you this error message (twice?!)

The cause of this error message should be fixed, and another message should be displayed (that the model doesn't contain any brushes)

Oddly enough when you create a brush, and then delete it, it doesn't show any error message

[LogicalError]

Partially fixed in commit 7449fff
Underlying cause for error message is still there (still needs to be fixed, even though it's harmless)
error message is no longer duplicated, and is clearer