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Remove metadata button is not disabled when there is not document opened. It produces a crash because it tries to obtain the widget of the metadata panel via Canvas View, which is non existing.

Attached is the backtrace. It happens in master and 0.64.1 branches.

Process: synfigstudio [12683]
Path: /Applications/SynfigStudio.app/Contents/MacOS/SynfigStudio
Identifier: synfigstudio
Version: ???
Code Type: X86-64 (Native)
Parent Process: launchd [146]
User ID: 501

Date/Time: 2013-10-07 17:00:19.968 +0200
OS Version: Mac OS X 10.8.4 (12E55)
Report Version: 10

Interval Since Last Report: 5215720 sec
Crashes Since Last Report: 8199
Per-App Crashes Since Last Report: 20
Anonymous UUID: 738EBCC0-5BB0-349F-6238-E5A843F59DA6

Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x00000000000005f0

VM Regions Near 0x5f0:
-->
__TEXT 0000000100000000-00000001003e9000 [ 4004K] r-x/rwx SM=COW /Applications/SynfigStudio.app/Contents/Resources/synfig/bin/synfigstudio

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 synfigstudio 0x00000001001e92c0 std::map<std::string, Gtk::Widget*, std::less<std::string>, std::allocator<std::pair<std::string const, Gtk::Widget*> > >::operator[](std::string const&) + 72
1 synfigstudio 0x00000001001bfc12 studio::CanvasView::get_ext_widget(std::string const&) + 16
2 synfigstudio 0x000000010005ce61 studio::Dock_MetaData::on_delete_pressed() + 47
3 libglibmm-2.4.1.dylib 0x000000010159670f Glib::SignalProxyNormal::slot0_void_callback(_GObject*, void*) + 63
4 libgobject-2.0.0.dylib 0x0000000101fd47a1 g_closure_invoke + 237
5 libgobject-2.0.0.dylib 0x0000000101fe7874 signal_emit_unlocked_R + 2468
6 libgobject-2.0.0.dylib 0x0000000101fe85ea g_signal_emit_valist + 2845
7 libgobject-2.0.0.dylib 0x0000000101fe8aa0 g_signal_emit_by_name + 310
8 libgobject-2.0.0.dylib 0x0000000101fd494b _g_closure_invoke_va + 254
9 libgobject-2.0.0.dylib 0x0000000101fe7fb7 g_signal_emit_valist + 1258
10 libgobject-2.0.0.dylib 0x0000000101fe8952 g_signal_emit + 134
11 libgtk-x11-2.0.0.dylib 0x000000010165ad1f gtk_real_button_released + 51
12 libgobject-2.0.0.dylib 0x0000000101fd494b _g_closure_invoke_va + 254
13 libgobject-2.0.0.dylib 0x0000000101fe7fb7 g_signal_emit_valist + 1258
14 libgobject-2.0.0.dylib 0x0000000101fe8952 g_signal_emit + 134
15 libgtk-x11-2.0.0.dylib 0x000000010165aad7 gtk_button_button_release + 15
16 libgtk-x11-2.0.0.dylib 0x00000001016eee44 _gtk_marshal_BOOLEAN__BOXED + 159
17 libgobject-2.0.0.dylib 0x0000000101fd47a1 g_closure_invoke + 237
18 libgobject-2.0.0.dylib 0x0000000101fe7728 signal_emit_unlocked_R + 2136
19 libgobject-2.0.0.dylib 0x0000000101fe8653 g_signal_emit_valist + 2950
20 libgobject-2.0.0.dylib 0x0000000101fe8952 g_signal_emit + 134
21 libgtk-x11-2.0.0.dylib 0x00000001017e52ed gtk_widget_event_internal + 557
22 libgtk-x11-2.0.0.dylib 0x00000001016ed1d0 gtk_propagate_event + 380
23 libgtk-x11-2.0.0.dylib 0x00000001016ece45 gtk_main_do_event + 941
24 libgdk-x11-2.0.0.dylib 0x0000000101ab012c gdk_event_dispatch + 81
25 libglib-2.0.0.dylib 0x000000010211a565 g_main_context_dispatch + 303
26 libglib-2.0.0.dylib 0x000000010211a86d g_main_context_iterate + 420
27 libglib-2.0.0.dylib 0x000000010211aaf2 g_main_loop_run + 256
28 libgtk-x11-2.0.0.dylib 0x00000001016ec7d5 gtk_main + 179
29 synfigstudio 0x0000000100002b73 main + 1251
30 synfigstudio 0x0000000100002664 start + 52