DB Error after adding custom field

[MLindenhofer]

Hi,
CMS: SilverStripe 3.7.3, PHP: 7.2

I followed the instructions from the docs to add a custom field - in my case a CheckboxField "TermAgreement":

private static $db = [
    	...
	   'TermAgreement' => 'Boolean',
      ...
    ];

 public function updateMemberFormFields($fields) {
 
		$TermAgreementField = CheckboxField::create('TermAgreement', 'I agree that all my comments and articles, if editorially accepted for publication, shall be licensed under the CC-BY Creative Commons Attribution License 4.0.');
		$fields->push($TermAgreementField);
    }

but when I do a dev/build?flush to add the new MemberField to the DB too this Error is thrown:

[User Error] Uncaught SS_DatabaseException: Couldn't run query: SELECT DISTINCT "Member"."ClassName", "Member"."LastEdited", "Member"."Created", "Member"."FirstName", "Member"."Surname", "Member"."Email", "Member"."TempIDHash", "Member"."TempIDExpired", "Member"."Password", "Member"."RememberLoginToken", "Member"."NumVisit", "Member"."LastVisited", "Member"."AutoLoginHash", "Member"."AutoLoginExpired", "Member"."PasswordEncryption", "Member"."Salt", "Member"."PasswordExpiry", "Member"."LockedOutUntil", "Member"."Locale", "Member"."FailedLoginCount", "Member"."DateFormat", "Member"."TimeFormat", "Member"."Degree", "Member"."Institution", "Member"."TermAgreement", "Member"."Position", "Member"."OrcidID", "Member"."ContactDetails", "Member"."AreaOfExpertise", "Member"."References", "Member"."ReviewStatusRequested", "Member"."Verification", "Member"."IsPublic", "Member"."HasPublished", "Member"."ValidationKey", "Member"."NeedsValidation", "Member"."NeedsApproval", "Member"."PublicFieldsRaw", "Member"."ProfileImageID", "Member"."ProfilePageID", "Member"."ID", CASE WHEN "Member"."ClassName" IS NOT NULL THEN "Member"."ClassName" ELSE 'Member' END AS "RecordClassName" FROM "Member" WHERE ("Member"."ID" = ?) ORDER BY "Member"."Surname" ASC, "Member"."FirstName" ASC LIMIT 1 Unknown column 'Member.TermAgreement' in 'field list'
GET /dev/build?devbuildtoken=558c54c2cef98dca1dec0ea5056c0d5f
Line 55 in /vagrant/www/framework/model/connect/DBConnector.php

Reinstalling the module, each time I want to add a CustomField like #161 can't be the only solution? What can I do? Help please,

Thank you