Fix task cleanup

Author: pipe01

src/displayapp/screens/Pawn.cpp

@@ -221,7 +221,9 @@ Pawn::Pawn() {
 }
 
 Pawn::~Pawn() {
-  lv_task_del(taskRefresh);
+  if (taskRefresh)
+    lv_task_del(taskRefresh);
+
   lv_obj_clean(lv_scr_act());
 
   amx_Cleanup(&amx);

src/displayapp/screens/Pawn.h

@@ -21,7 +21,7 @@ namespace Pinetime {
         AMX amx;
         int refresh_index;
 
-        lv_task_t* taskRefresh;
+        lv_task_t* taskRefresh = 0;
       };
     }