When toggling a channel-configs the following happens:
1. TrackExportChannelSelector::update_config
2. ExportProfileManager::clear_channel_configs
3. RouteExportChannel::create_from_route
[...] CriticalSelectionChanged
4. ExportDialog::update_warnings_and_example_filename
5. ARDOUR::ExportProfileManager::get_warnings
6. ARDOUR::ExportProfileManager::build_filenames
7. ARDOUR::ExportFilename::set_channel_config
[...]
Step 3 creates a RouteExportChannel, Step 7 retains a
shared-pointer to it in the config's ExportFilenamePtr.
When toggling another channel: step 2 release the reference
and step 3 creates a new RouteExportChannel.
This new channel reuses the existing Route::_capture_processor (!).
Now Step 7 releases the first RouteExportChannel in order
to get_warnings for the new one. Since ExportFilenamePtr holds the
the last reference, the d'tor of RouteExportChannel runs,
which removes the capture-processor from the route.
The newly created RouteExportChannel now has a reference
to a CaptureProcessor that is not in the signal-flow.
Note: the order of adding/removing RouteExportChannel matters.
With a two track session:
* Disable "with track/bus processing
* Enable Track 1
* Enable Track 2
* Disable Track 1
* Enable Track 1 // << Track 2 becomes silent
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