Fix relation field unknown target object (#13129)

Fixes https://github.com/twentyhq/twenty/issues/12867

Issue:
when you have a variable `toto` which is: `Record<string, MyType>` and
you do toto['xxx'], this will be typed as `MyType` instead of `MyType |
undefined`

Solutions:
- activate `noUncheckedIndexedAccess` check in tsconfig, this is the
preferred solution but will take time to get there (this raises 600+
errors)
- use a Map: cf https://github.com/twentyhq/twenty/pull/13125/files
- set the type to Partial<Record<string, MyType>>. Drawback is that when
you do Object.values(toto), you'll get `Array<MyType | undefined>`.
Hence why we have to filter these behind


<img width="1512" alt="image"
src="https://github.com/user-attachments/assets/d0a0bfed-c441-4e53-84c2-2da98ccbcf50"
/>

packages/twenty-server/src/engine/metadata-modules/field-metadata/services/field-metadata-relation.service.ts

@@ -166,6 +166,13 @@ export class FieldMetadataRelationService {
             relationCreationPayload.targetObjectMetadataId
           ];
 
+        if (!isDefined(objectMetadataTarget)) {
+          throw new FieldMetadataException(
+            `Object metadata relation target not found for relation creation payload`,
+            FieldMetadataExceptionCode.FIELD_METADATA_RELATION_MALFORMED,
+          );
+        }
+
         validateFieldNameAvailabilityOrThrow(
           computedMetadataNameFromLabel,
           objectMetadataTarget,

packages/twenty-server/src/engine/metadata-modules/field-metadata/services/field-metadata.service.ts

@@ -107,7 +107,9 @@ export class FieldMetadataService extends TypeOrmQueryService<FieldMetadataEntit
 
     let existingFieldMetadata: FieldMetadataInterface | undefined;
 
-    for (const objectMetadataItem of Object.values(objectMetadataMaps.byId)) {
+    for (const objectMetadataItem of Object.values(
+      objectMetadataMaps.byId,
+    ).filter(isDefined)) {
       const fieldMetadata = objectMetadataItem.fieldsById[id];
 
       if (fieldMetadata) {
@@ -126,6 +128,13 @@ export class FieldMetadataService extends TypeOrmQueryService<FieldMetadataEntit
     const objectMetadataItemWithFieldMaps =
       objectMetadataMaps.byId[existingFieldMetadata.objectMetadataId];
 
+    if (!isDefined(objectMetadataItemWithFieldMaps)) {
+      throw new FieldMetadataException(
+        'Object metadata does not exist',
+        FieldMetadataExceptionCode.OBJECT_METADATA_NOT_FOUND,
+      );
+    }
+
     const queryRunner = this.coreDataSource.createQueryRunner();
 
     await queryRunner.connect();
@@ -703,7 +712,7 @@ export class FieldMetadataService extends TypeOrmQueryService<FieldMetadataEntit
     isRemoteCreation,
   }: {
     createdFieldMetadataItems: FieldMetadataEntity[];
-    objectMetadataMap: Record<string, ObjectMetadataItemWithFieldMaps>;
+    objectMetadataMap: ObjectMetadataMaps['byId'];
     isRemoteCreation: boolean;
   }): Promise<WorkspaceMigrationTableAction[]> {
     if (isRemoteCreation) {
@@ -726,10 +735,18 @@ export class FieldMetadataService extends TypeOrmQueryService<FieldMetadataEntit
         }
       }
 
+      const objectMetadata =
+        objectMetadataMap[createdFieldMetadata.objectMetadataId];
+
+      if (!isDefined(objectMetadata)) {
+        throw new FieldMetadataException(
+          'Object metadata does not exist',
+          FieldMetadataExceptionCode.OBJECT_METADATA_NOT_FOUND,
+        );
+      }
+
       migrationActions.push({
-        name: computeObjectTargetTable(
-          objectMetadataMap[createdFieldMetadata.objectMetadataId],
-        ),
+        name: computeObjectTargetTable(objectMetadata),
         action: WorkspaceMigrationTableActionType.ALTER,
         columns: this.workspaceMigrationFactory.createColumnActions(
           WorkspaceMigrationColumnActionType.CREATE,