Fix potential crash when clearing hold highlight (#3241)

A delayed clear() could reset hold_pos while a onHold/onHoldPan/
onHoldRelease is in progress, resulting in hold_pos becoming nil and
a crash.

frontend/apps/reader/modules/readerhighlight.lua

@@ -110,7 +110,23 @@ function ReaderHighlight:genHighlightDrawerMenu()
     }
 end
 
-function ReaderHighlight:clear()
+-- Returns a unique id, that can be provided on delayed call to :clear(id)
+-- to ensure current highlight has not already been cleared, and that we
+-- are not going to clear a new highlight
+function ReaderHighlight:getClearId()
+    self.clear_id = TimeVal.now() -- can act as a unique id
+    return self.clear_id
+end
+
+function ReaderHighlight:clear(clear_id)
+    if clear_id then -- should be provided by delayed call to clear()
+        if clear_id ~= self.clear_id then
+            -- if clear_id is no more valid, highlight has already been
+            -- cleared since this clear_id was given
+            return
+        end
+    end
+    self.clear_id = nil -- invalidate id
     if self.ui.document.info.has_pages then
         self.view.highlight.temp = {}
     else
@@ -223,6 +239,7 @@ end
 function ReaderHighlight:onHold(arg, ges)
     -- disable hold gesture if highlighting is disabled
     if self.view.highlight.disabled then return true end
+    self:clear() -- clear previous highlight (delayed clear may not have done it yet)
     self.hold_pos = self.view:screenToPageTransform(ges.pos)
     logger.dbg("hold position in page", self.hold_pos)
     if not self.hold_pos then

frontend/ui/widget/dictquicklookup.lua

@@ -613,8 +613,9 @@ function DictQuickLookup:onClose()
     if self.highlight then
         -- delay unhighlight of selection, so we can see where we stopped when
         -- back from our journey into dictionary or wikipedia
+        local clear_id = self.highlight:getClearId()
         UIManager:scheduleIn(0.5, function()
-            self.highlight:clear()
+            self.highlight:clear(clear_id)
         end)
     end
     return true
@@ -626,8 +627,9 @@ function DictQuickLookup:onHoldClose(no_clear)
         local window = self.window_list[i]
         -- if one holds a highlight, let's clear it like in onClose()
         if window.highlight and not no_clear then
-            UIManager:scheduleIn(1, function()
-                window.highlight:clear()
+            local clear_id = window.highlight:getClearId()
+            UIManager:scheduleIn(0.5, function()
+                window.highlight:clear(clear_id)
             end)
         end
         UIManager:close(window)