[fix] Don't crash when trying to go to deleted folders (#4110)

frontend/apps/filemanager/filemanager.lua

@@ -47,6 +47,7 @@ local function restoreScreenMode()
 end
 
 local function truncatePath(text)
+    if not text then return "" end
     local screen_width = Screen:getWidth()
     local face = Font:getFace("xx_smallinfofont")
     -- we want to truncate text on the left, so work with the reverse of text (which is fine as we don't use kerning)

frontend/apps/filemanager/filemanagerutil.lua

@@ -21,6 +21,7 @@ function filemanagerutil.getDefaultDir()
 end
 
 function filemanagerutil.abbreviate(path)
+    if not path then return "" end
     local home_dir_name = G_reader_settings:readSetting("home_dir_display_name")
     if home_dir_name ~= nil then
         local home_dir = G_reader_settings:readSetting("home_dir") or filemanagerutil.getDefaultDir()