Fix crash closing/reopening sprites when "keep closed sprite data in memory" is disabled

src/app/crash/backup_observer.cpp

@@ -98,6 +98,9 @@ void BackupObserver::onRemoveDocument(Doc* doc)
     base::remove_from_container(m_documents, doc);
   }
   if (doc->needsBackup() &&
+      // If the document is already fully backed up, we don't need to
+      // add it to the background thread to create its backup
+      !doc->isFullyBackedUp() &&
       // If the backup is disabled, we don't need it (e.g. when the
       // document is destroyed from a script with Sprite:close(), the
       // backup is disabled)

src/app/doc.cpp

@@ -81,6 +81,8 @@ void Doc::setContext(Context* ctx)
 
   m_ctx = ctx;
   if (ctx) {
+    DOC_TRACE("DOC: Removing as fully backed up", this);
+
     // Remove the flag that indicates that this doc is fully backed
     // up, because now we are inside a context, so the user can change
     // it again and the backup will be outdated.
@@ -260,6 +262,8 @@ void Doc::setInhibitBackup(const bool inhibitBackup)
 
 void Doc::markAsBackedUp()
 {
+  DOC_TRACE("DOC: Mark as fully backed up", this);
+
   m_flags |= kFullyBackedUp;
 }