[Flowchart] Rendering fails when an edge targets a subgraph ID

[Jonesonlilei]

Description

Description

Rendering a flowchart fails when an edge starts or ends at a subgraph ID.

The diagram can be parsed successfully, but rendering fails during the Dagre layout phase with:

TypeError: Cannot set properties of undefined (setting 'rank')

### Steps to reproduce

```mermaid
flowchart TD
    Check --> Decision
    Decision -- Then --> ThenSequence
    Decision -- Else --> ElseSequence
    OuterSequence --> Chat

    subgraph OuterSequence[Sequence]
        subgraph InnerSequence[Sequence]
            Check[Check]
            Decision{Decision}

            subgraph ThenSequence[Then]
                Click[Click] --> Extract[Extract]
            end

            subgraph ElseSequence[Else]
                Execute[Execute] --> GetContent[Get Content]
            end
        end
    end

Screenshots

No response

Code Sample

Setup

• Mermaid version:11.12.3
• Browser and Version: [Chrome:152.0.7977.65]

Suggested Solutions

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Additional Context

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