Fix testTabletInterface

src/server/tablet_interface.cpp

diff --git a/src/server/tablet_interface.cpp b/src/server/tablet_interface.cpp
index 554dd1c..b296323 100644
--- a/src/server/tablet_interface.cpp
+++ b/src/server/tablet_interface.cpp
@@ -155,6 +155,9 @@
 
     wl_resource *targetResource()
     {
+        if (!m_surface)
+            return nullptr;
+
         ClientConnection *client = m_surface->client();
         const Resource *r = resourceMap().value(*client);
         return r ? r->handle : nullptr;