Fix crash applying a filter w/all layers locked (fix #1610)

The m_transaction field of FilterManagerImpl::commitTransaction() was
nullptr because FilterManagerImpl::applyToTarget() was returning without
creating the m_transaction.

src/app/commands/filters/filter_manager_impl.cpp

@@ -234,8 +234,11 @@ void FilterManagerImpl::applyToTarget()
                          m_site.frame(),
                          (m_target & TARGET_ALL_FRAMES) == TARGET_ALL_FRAMES,
                          true); // we will write in each image
-  if (images.empty() && !paletteChange)
+  if (images.empty() && !paletteChange) {
+    // We don't have images/palette changes to do (there will not be a
+    // transaction).
     return;
+  }
 
   // Initialize writting operation
   ContextReader reader(m_context);
@@ -280,10 +283,16 @@ void FilterManagerImpl::applyToTarget()
   m_oldPalette.reset(nullptr);
 }
 
+bool FilterManagerImpl::isTransaction() const
+{
+  return m_transaction != nullptr;
+}
+
+// This must be executed in the main UI thread.
+// Check Transaction::commit() comments.
 void FilterManagerImpl::commitTransaction()
 {
-  // This must be executed in the main UI thread.
-  // Check Transaction::commit() comments.
+  ASSERT(m_transaction);
   m_transaction->commit();
 }
 

src/app/commands/filters/filter_manager_impl.h

@@ -83,6 +83,7 @@ namespace app {
     void end();
     bool applyStep();
     void applyToTarget();
+    bool isTransaction() const;
     void commitTransaction();
 
     app::Document* document();

src/app/commands/filters/filter_worker.cpp

@@ -16,6 +16,7 @@
 #include "app/modules/editors.h"
 #include "app/modules/gui.h"
 #include "app/ui/editor/editor.h"
+#include "app/ui/status_bar.h"
 #include "base/mutex.h"
 #include "base/scoped_lock.h"
 #include "base/thread.h"
@@ -104,7 +105,7 @@ void FilterWorker::run()
 
   {
     scoped_lock lock(m_mutex);
-    if (m_done)
+    if (m_done && m_filterMgr->isTransaction())
       m_filterMgr->commitTransaction();
     else
       m_cancelled = true;
@@ -117,6 +118,10 @@ void FilterWorker::run()
     Console console;
     console.printf("A problem has occurred.\n\nDetails:\n%s", m_error.c_str());
   }
+  else if (m_cancelled && !m_filterMgr->isTransaction()) {
+    StatusBar::instance()
+      ->showTip(2500, "No unlocked layers to apply filter");
+  }
 }
 
 // Called by FilterManagerImpl to informate the progress of the filter.

src/app/document_undo.cpp

@@ -228,6 +228,7 @@ const undo::UndoState* DocumentUndo::nextRedo() const
 
 void DocumentUndo::onDeleteUndoState(undo::UndoState* state)
 {
+  ASSERT(state);
   Cmd* cmd = STATE_CMD(state);
 
   UNDO_TRACE("UNDO: Deleting undo state <%s> of %s from %s\n",