Scripting: Fixed crash after using tiled.project

Crash caused by a double-deletion of the EditableProject, since both
the Project as well as the ProjectDocument classes were deleting it
from their destructors (in the Object and Document superclasses,
respectively).

This issue was introduced in 4244060.

src/tiled/projectdocument.cpp

@@ -37,6 +37,13 @@ ProjectDocument::ProjectDocument(std::unique_ptr<Project> project, QObject *pare
             this, [this] { mProject->save(); });
 }
 
+ProjectDocument::~ProjectDocument()
+{
+    // The Editable needs to be deleted before the Project, otherwise ~Object()
+    // will delete it, whereas the editable is actually owned by the Document.
+    mEditable.reset();
+}
+
 QString ProjectDocument::displayName() const
 {
     return mProject->fileName();

src/tiled/projectdocument.h

@@ -32,6 +32,7 @@ class ProjectDocument : public Document
 
 public:
     ProjectDocument(std::unique_ptr<Project> project, QObject *parent = nullptr);
+    ~ProjectDocument() override;
 
     QString displayName() const override;
     FileFormat *writerFormat() const override;