Portainer panics when starting off an existing EE database without data migration

[deviantony]

Bug description

Starting Portainer CE from an existing EE database without going through the database migration steps will create a Go panic.

Expected behavior

Portainer should display an error message if it is unable to go through the database migration process and provide a link to the database migration documentation.

Portainer Logs

2020/11/09 02:21:30 Migrating database from version 1 to 25.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x5e08ca]

goroutine 1 [running]:
github.com/boltdb/bolt.(*Bucket).Cursor(...)
	/tmp/go/pkg/mod/github.com/boltdb/bolt@v1.3.1/bucket.go:91
github.com/portainer/portainer/api/bolt/migrator.(*Migrator).retrieveLegacyResourceControls.func1(0xc000130ee0, 0x0, 0xc000130ee0)
	/home/vsts/work/1/s/api/bolt/migrator/migrate_dbversion1.go:66 +0xaa
github.com/boltdb/bolt.(*DB).View(0xc00000a960, 0xc0005359b8, 0x0, 0x0)
	/tmp/go/pkg/mod/github.com/boltdb/bolt@v1.3.1/db.go:629 +0xa8
github.com/portainer/portainer/api/bolt/migrator.(*Migrator).retrieveLegacyResourceControls(0xc000535b80, 0x3d, 0x40, 0x3d, 0x0, 0x0)
	/home/vsts/work/1/s/api/bolt/migrator/migrate_dbversion1.go:64 +0x82
github.com/portainer/portainer/api/bolt/migrator.(*Migrator).updateResourceControlsToDBVersion2(0xc000535b80, 0x0, 0x0)
	/home/vsts/work/1/s/api/bolt/migrator/migrate_dbversion1.go:10 +0x40
github.com/portainer/portainer/api/bolt/migrator.(*Migrator).Migrate(0xc000535b80, 0x2a, 0xc000535b60)
	/home/vsts/work/1/s/api/bolt/migrator/migrator.go:105 +0x7e6
github.com/portainer/portainer/api/bolt.(*Store).MigrateData(0xc0000b81c0, 0x0, 0x0)
	/home/vsts/work/1/s/api/bolt/datastore.go:157 +0x3d1
main.initDataStore(0x19dc1cc, 0x5, 0x1cb0100, 0xc0001d0f60, 0xc0000a2200, 0x5)
	/home/vsts/work/1/s/api/cmd/portainer/main.go:67 +0x156
main.main()
	/home/vsts/work/1/s/api/cmd/portainer/main.go:340 +0x99

Steps to reproduce the issue:

1. Deploy an instance of Portainer EE
2. Stop that instance
3. Deploy an instance of Portainer CE reusing the data from the EE instance

Technical details:

• Portainer version: 2.0-CE