Applied Element Template is not unlinked, when changing to not supported element type

Describe the Enhancement

When applying an task-specific element template to a task of a specific type (e.g., ServiceTask) and then changing the type of the respective task (e.g., to UserTask), the element template is still present. This is IMO an inconsistent state, since the element template configuration would not allow that template to be applied to a task other than ServiceTask.

Steps to Reproduce

1. Setup the following example element template

{
    "name": "Mail Task",
    "id": "com.camunda.example.MailTask",
    "appliesTo": [
      "bpmn:ServiceTask"
    ],
    "properties": [
      {
        "label": "Implementation Type",
        "type": "String",
        "value": "com.mycompany.MailTaskImpl",
        "editable": false,
        "binding": {
          "type": "property",
          "name": "camunda:class"
        }
      },
      {
        "label": "Sender",
        "type": "String",
        "binding": {
          "type": "camunda:inputParameter",
          "name": "sender"
        },
        "constraints": {
          "notEmpty": true,
          "pattern": {
            "value": "^[A-z0-9._%+-]+@[A-z0-9.-]+\\.[A-z]{2,}$",
            "message": "Must be a valid email."
          }
        }
      },
      {
        "label": "Receivers",
        "type": "String",
        "binding": {
          "type": "camunda:inputParameter",
          "name": "receivers"
        },
        "constraints": {
          "notEmpty": true
        }
      },
      {
        "label": "Template",
        "description": "By the way, you can use <a href=\"https://freemarker.apache.org/\">freemarker templates</a> here",
        "value": "Hello ${firstName}!",
        "type": "Text",
        "binding": {
          "type": "camunda:inputParameter",
          "name": "messageBody",
          "scriptFormat": "freemarker"
        },
        "constraints": {
          "notEmpty": true
        }
      },
      {
        "label": "Result Status",
        "description": "The process variable to which to assign the send result to",
        "type": "String",
        "value": "mailSendResult",
        "binding": {
          "type": "camunda:outputParameter",
          "source": "${ resultStatus }"
        }
      },
      {
        "label": "Send Async?",
        "type": "Boolean",
        "value": true,
        "binding": {
          "type": "property",
          "name": "camunda:asyncBefore"
        }
      }
    ]
  }

1. Apply the template to a serviceTask
2. Change the task type to userTask
3. => The element template is still applied

Expected Behavior

The element template shall be unlinked, if not supporting the respective new task type.

Why

If we don’t have this, the user would get to an inconsistent state of the diagram (according to the element template configuration)

Environment

• OS: Arch Linux Manjaro
• Camunda Modeler Version: develop branch + develop branch bpmn-js-properties-panel on 6th October

[philippfromme]

The solution I’ve discussed with @azeghers is:

• when the template cannot be found it will be unlinked
• when the template can be found it will be unlinked if it can’t be applied to the new element

CodeSandbox example: https://codesandbox.io/s/restricting-camundamodelertemplate-nyrtp?file=/src/index.js

[pinussilvestrus]

Closed via https://github.com/camunda/camunda-modeler/commit/d03c2a69a1adc4dcf236afd049b195e40e098a77