Fix #145259: Grease Pencil: Bail when no frame is selected for SVG export

Previously `SVGExporter::export_scene` can work on a empty `frames`
index mask, causing crash. Now it will exit early when no frames are
selected, and it will show a warning to the user.

Pull Request: https://projects.blender.org/blender/blender/pulls/145286

source/blender/editors/io/io_grease_pencil.cc

@@ -295,6 +295,7 @@ static wmOperatorStatus grease_pencil_export_svg_invoke(bContext *C,
 static wmOperatorStatus grease_pencil_export_svg_exec(bContext *C, wmOperator *op)
 {
   using blender::io::grease_pencil::ExportParams;
+  using blender::io::grease_pencil::ExportStatus;
   using blender::io::grease_pencil::IOContext;
 
   Scene *scene = CTX_data_scene(C);
@@ -337,11 +338,25 @@ static wmOperatorStatus grease_pencil_export_svg_exec(bContext *C, wmOperator *o
                                stroke_sample};
 
   WM_cursor_wait(true);
-  const bool done = blender::io::grease_pencil::export_svg(io_context, params, *scene, filepath);
+  ExportStatus status = blender::io::grease_pencil::export_svg(
+      io_context, params, *scene, filepath);
   WM_cursor_wait(false);
 
-  if (!done) {
-    BKE_report(op->reports, RPT_WARNING, "Unable to export SVG");
+  switch (status) {
+    case ExportStatus::Ok:
+      break;
+    case ExportStatus::InvalidActiveObjectType:
+      BKE_report(op->reports, RPT_WARNING, "Active object is not a Grease Pencil object");
+      break;
+    case ExportStatus::NoFramesSelected:
+      BKE_report(op->reports, RPT_WARNING, "No frames selected in the Grease Pencil object");
+      break;
+    case ExportStatus::FileWriteError:
+      BKE_reportf(op->reports, RPT_WARNING, "Error during file write for \"%s\"", filepath);
+      break;
+    case ExportStatus::UnknownError:
+      BLI_assert_unreachable();
+      break;
   }
 
   return OPERATOR_FINISHED;

source/blender/io/grease_pencil/grease_pencil_io.hh

@@ -45,6 +45,14 @@ struct ImportParams {
   bool recenter_bounds = false;
 };
 
+enum class ExportStatus : int8_t {
+  Ok = 0,
+  NoFramesSelected,
+  InvalidActiveObjectType,
+  FileWriteError,
+  UnknownError,
+};
+
 struct ExportParams {
   /* Object to be exported. */
   enum class SelectMode {
@@ -74,10 +82,10 @@ struct ExportParams {
 };
 
 bool import_svg(const IOContext &context, const ImportParams &params, StringRefNull filepath);
-bool export_svg(const IOContext &context,
-                const ExportParams &params,
-                Scene &scene,
-                StringRefNull filepath);
+ExportStatus export_svg(const IOContext &context,
+                        const ExportParams &params,
+                        Scene &scene,
+                        StringRefNull filepath);
 bool export_pdf(const IOContext &context,
                 const ExportParams &params,
                 Scene &scene,

source/blender/io/grease_pencil/intern/grease_pencil_io_export_svg.cc

@@ -106,7 +106,7 @@ class SVGExporter : public GreasePencilExporter {
 
   pugi::xml_document main_doc_;
 
-  bool export_scene(Scene &scene, StringRefNull filepath);
+  ExportStatus export_scene(Scene &scene, StringRefNull filepath);
   void export_grease_pencil_objects(pugi::xml_node node, int frame_number);
   void export_grease_pencil_layer(pugi::xml_node node,
                                   const Object &object,
@@ -146,7 +146,7 @@ std::string SVGExporter::get_node_uuid_string()
   return id;
 }
 
-bool SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
+ExportStatus SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
 {
   this->_node_uuid = 0;
 
@@ -160,7 +160,8 @@ bool SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
 
       this->export_grease_pencil_objects(main_node, frame_number);
 
-      return this->write_to_file(filepath);
+      const bool write_success = this->write_to_file(filepath);
+      return write_success ? ExportStatus::Ok : ExportStatus::FileWriteError;
     }
     case ExportParams::FrameMode::Selected:
     case ExportParams::FrameMode::Scene: {
@@ -172,6 +173,9 @@ bool SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
       IndexMaskMemory memory;
       if (selection_only) {
         const Object &ob_eval = *DEG_get_evaluated(context_.depsgraph, params_.object);
+        if (ob_eval.type != OB_GREASE_PENCIL) {
+          return ExportStatus::InvalidActiveObjectType;
+        }
         const GreasePencil &grease_pencil = *static_cast<GreasePencil *>(ob_eval.data);
         frames = IndexMask::from_predicate(
             frames, GrainSize(1024), memory, [&](const int frame_number) {
@@ -179,6 +183,10 @@ bool SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
             });
       }
 
+      if (frames.is_empty()) {
+        return ExportStatus::NoFramesSelected;
+      }
+
       this->prepare_render_params(scene, frames.first());
 
       this->write_document_header();
@@ -209,11 +217,12 @@ bool SVGExporter::export_scene(Scene &scene, StringRefNull filepath)
 
       this->write_animation_node(main_node, frames, duration);
 
-      return this->write_to_file(filepath);
+      const bool write_success = this->write_to_file(filepath);
+      return write_success ? ExportStatus::Ok : ExportStatus::FileWriteError;
     }
     default:
       BLI_assert_unreachable();
-      return false;
+      return ExportStatus::UnknownError;
   }
 }
 
@@ -572,10 +581,10 @@ bool SVGExporter::write_to_file(StringRefNull filepath)
   return result;
 }
 
-bool export_svg(const IOContext &context,
-                const ExportParams &params,
-                Scene &scene,
-                StringRefNull filepath)
+ExportStatus export_svg(const IOContext &context,
+                        const ExportParams &params,
+                        Scene &scene,
+                        StringRefNull filepath)
 {
   SVGExporter exporter(context, params);
   return exporter.export_scene(scene, filepath);