[18.0][FIX] mis_builder: guard against falsy drilldown action

[dnplkndll]

Description

Clicking a computed KPI cell (e.g. profit - loss) in the MIS report
widget triggers a JS error:

No view found for act_window action undefined

Root cause

mis_report_instance.drilldown() returns False when the expression
has no account variable (AEP.has_account_var() is False for computed
expressions). The JS handler passes this falsy value directly to
this.action.doAction().

Fix

Add a truthy check before calling doAction().

Closes #776

[OCA-git-bot]

Hi @sbidoul,
some modules you are maintaining are being modified, check this out!

[sbidoul]

Hi, this change looks good but I really wonder why such a cell would be clickable in the first place.

For instance with the demo report you can see on runboat, the Total row is not clickable.

[dnplkndll]

Closing this — the root cause turned out to be a migration issue, not a missing JS guard.

After migrating from 17.0 to 18.0, source_aml_model_id (a stored computed field introduced in 18.0) is NULL on all existing mis.report.instance.period records. This causes drilldown() to return an action with res_model: False, which the JS then fails on with "No view found for act_window action undefined".

On a clean 18.0 install, source_aml_model_id is computed correctly and drilldown works fine — no JS guard needed.

The proper fix is a migration script to recompute the field: #780 / ledoent#3