"Duplicate entry '' for key" in Grails Database Migration using hasOne - mysql

Question

Using Grails Database Migration Plugin 1.4 I want have the following structure of my existing database:
class StickerGroup {
}
Now I added the class Company and updated the class StickerGroup:
class StickerGroup {
static constraints = { company nullable: true, unique: true }
static belongsTo = [company: Company]
}
class Company {
static hasOne = [stickerGroup: StickerGroup]
}
I created the migration file and then I perform grails dbm-update. After that I get the following error log:
2015-01-20 15:03:49,549 [main] ERROR liquibase - Change Set 2015-01-20-update-Company.groovy::1421762562430-2::mg (generated) failed. Error: Error executing SQL ALTER TABLE `sticker_group` ADD `company_id` VARCHAR(10) NOT NULL UNIQUE: Duplicate entry '' for key 'company_id'
Message: Error executing SQL ALTER TABLE `sticker_group` ADD `company_id` VARCHAR(10) NOT NULL UNIQUE: Duplicate entry '' for key 'company_id'
Line | Method
->> 62 | execute in liquibase.executor.jvm.JdbcExecutor
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| 104 | execute in ''
| 1091 | execute . . . . . in liquibase.database.AbstractDatabase
| 1075 | executeStatements in ''
| 317 | execute . . . . . in liquibase.changelog.ChangeSet
| 27 | visit in liquibase.changelog.visitor.UpdateVisitor
| 58 | run . . . . . . . in liquibase.changelog.ChangeLogIterator
| 114 | update in liquibase.Liquibase
| 26 | doCall . . . . . in DbmUpdate$_run_closure1_closure2
| 59 | doCall in _DatabaseMigrationCommon_groovy$_run_closure2_closure11
| 133 | executeInSession in grails.plugin.databasemigration.MigrationUtils
| 51 | doCall in _DatabaseMigrationCommon_groovy$_run_closure2
^ 25 | doCall . . . . . in DbmUpdate$_run_closure1
Caused by MySQLIntegrityConstraintViolationException: Duplicate entry '' for key 'company_id'
->> 377 | handleNewInstance in com.mysql.jdbc.Util
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| 360 | getInstance in ''
| 971 | createSQLException in com.mysql.jdbc.SQLError
| 3887 | checkErrorPacket in com.mysql.jdbc.MysqlIO
| 3823 | checkErrorPacket in ''
| 2435 | sendCommand in ''
| 2582 | sqlQueryDirect . in ''
| 2526 | execSQL in com.mysql.jdbc.ConnectionImpl
| 2484 | execSQL . . . . . in ''
| 848 | execute in com.mysql.jdbc.StatementImpl
| 742 | execute . . . . . in ''
| 92 | doInStatement in liquibase.executor.jvm.JdbcExecutor$1ExecuteStatementCallback
| 55 | execute . . . . . in liquibase.executor.jvm.JdbcExecutor
| 104 | execute in ''
| 1091 | execute . . . . . in liquibase.database.AbstractDatabase
| 1075 | executeStatements in ''
| 317 | execute . . . . . in liquibase.changelog.ChangeSet
| 27 | visit in liquibase.changelog.visitor.UpdateVisitor
| 58 | run . . . . . . . in liquibase.changelog.ChangeLogIterator
| 114 | update in liquibase.Liquibase
| 26 | doCall . . . . . in DbmUpdate$_run_closure1_closure2
| 59 | doCall in _DatabaseMigrationCommon_groovy$_run_closure2_closure11
| 133 | executeInSession in grails.plugin.databasemigration.MigrationUtils
| 51 | doCall in _DatabaseMigrationCommon_groovy$_run_closure2
^ 25 | doCall . . . . . in DbmUpdate$_run_closure1
liquibase.exception.MigrationFailedException: Migration failed for change set 2015-01-20-update-Company.groovy::1421762562430-2::mg (generated):
Reason: liquibase.exception.DatabaseException: Error executing SQL ALTER TABLE `sticker_group` ADD `company_id` VARCHAR(10) NOT NULL UNIQUE: Duplicate entry '' for key 'company_id':
Caused By: Error executing SQL ALTER TABLE `sticker_group` ADD `company_id` VARCHAR(10) NOT NULL UNIQUE: Duplicate entry '' for key 'company_id':
Caused By: Duplicate entry '' for key 'company_id'
at liquibase.changelog.ChangeSet.execute(ChangeSet.java:347)
at liquibase.changelog.visitor.UpdateVisitor.visit(UpdateVisitor.java:27)
at liquibase.changelog.ChangeLogIterator.run(ChangeLogIterator.java:58)
at liquibase.Liquibase.update(Liquibase.java:114)
at DbmUpdate$_run_closure1_closure2.doCall(DbmUpdate:26)
at _DatabaseMigrationCommon_groovy$_run_closure2_closure11.doCall(_DatabaseMigrationCommon_groovy:59)
at grails.plugin.databasemigration.MigrationUtils.executeInSession(MigrationUtils.groovy:133)
at _DatabaseMigrationCommon_groovy$_run_closure2.doCall(_DatabaseMigrationCommon_groovy:51)
at DbmUpdate$_run_closure1.doCall(DbmUpdate:25)
Caused by: liquibase.exception.DatabaseException: Error executing SQL ALTER TABLE `sticker_group` ADD `company_id` VARCHAR(10) NOT NULL UNIQUE: Duplicate entry '' for key 'company_id'
at liquibase.executor.jvm.JdbcExecutor.execute(JdbcExecutor.java:62)
at liquibase.executor.jvm.JdbcExecutor.execute(JdbcExecutor.java:104)
at liquibase.database.AbstractDatabase.execute(AbstractDatabase.java:1091)
at liquibase.database.AbstractDatabase.executeStatements(AbstractDatabase.java:1075)
at liquibase.changelog.ChangeSet.execute(ChangeSet.java:317)
... 8 more
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '' for key 'company_id'
at com.mysql.jdbc.Util.handleNewInstance(Util.java:377)
at com.mysql.jdbc.Util.getInstance(Util.java:360)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:971)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3887)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3823)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2435)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2582)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2526)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2484)
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:848)
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:742)
at liquibase.executor.jvm.JdbcExecutor$1ExecuteStatementCallback.doInStatement(JdbcExecutor.java:92)
at liquibase.executor.jvm.JdbcExecutor.execute(JdbcExecutor.java:55)
... 12 more
How can I fix this error?