Using a WeakPtr to keep a reference to the active layer caused it to
be destroyed when the last tab was closed, which made the
m_layer == layer check in set_layer() return early since it was
already null. Because of this the LayerPropertiesWidget was never
disabled.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
