For Classic Era Hardcore PTR (toc 1.14.4) with Plater version v560.4-22-gb4b772d-Classic - DF v444 - 1.14.4
I'm getting the following Lua error when logging in. In addition, most of the functionality of the add-on seems to be broken at the moment.

2x Plater/libs/DF/picture.lua:335: attempt to call method 'SetGradientAlpha' (a nil value)
[string "@Plater/libs/DF/picture.lua"]:335: in function <Plater/libs/DF/picture.lua:269>
[string "=(tail call)"]: ?
[string "@Plater/libs/DF/cooltip.lua"]:255: in function createTooltipFrames' [string "@Plater/libs/DF/cooltip.lua"]:330: in function CreateCoolTip'
[string "@Plater/libs/DF/cooltip.lua"]:3651: in main chunk

Locals:
self =

{
DropDownCounter = 747047
CreateFrameContainer = defined @Plater/libs/DF/containers.lua:487
IsUnitTapDenied = defined @Plater/libs/DF/fw.lua:5128
NewLabel = defined @Plater/libs/DF/label.lua:255
SavedVars = {
}
AdjustmentSliderOptions = {
}
PanelCounter = 747047
MakeFunctionSecure = defined @Plater/libs/DF/fw.lua:5372
debug = false
LabelNameCounter = 747047
DefaultMetaFunctionsSet = {
}
GetCurrentSpec = defined @Plater/libs/DF/fw.lua:4353
alias_text_colors = {
}
ScriptHookMixin = {
}
BackdropUtil = {
}
RegisterScriptComm = defined @Plater/libs/DF/fw.lua:5070
UnitGroupRolesAssigned = defined @Plater/libs/DF/fw.lua:232
ShowTutorialAlertFrame = defined @Plater/libs/DF/fw.lua:2786
CooldownsExternals = {
}
GetSpecializationRole = defined @Plater/libs/DF/fw.lua:291
SpecListByClass = {
}
FRAMELEVEL_BACKGROUND = 150
GetDefaultColorList = defined @Plater/libs/DF/colors.lua:176
button_templates = {
}
HeaderMixin = {
}
FrameMixin = {
}
SplitTextInLines = defined @Plater/libs/DF/fw.lua:658
CreateSwitch = defined @Plater/libs/DF/slider.lua:872
font_templates = {
}
embeds = {
}
SetAnchor = defined @Plater/libs/DF/fw.lua:1445
GetDefaultBackdropColor = defined @Plater/libs/DF/fw.lua:65
trim = defined @Plater/libs/DF/fw.lua:1095
NewSplitBar = defined @Plater/libs/DF/split_bar.lua:716
LayeredRegionMetaFunctionsSet = {
}
FindClassForCooldown = defined @Plater/libs/DF/spells.lua:1269
ChartFrameMixin = {
}
TooltipHandlerMixin = {
}
DebugVisibility = defined @Plater/libs/DF/fw.lua:5382
MakeStringFromSpellId = defined @Plater/libs/DF/fw.lua:974
BarNameCounter = 747047
CreateTexture = defined @Plater/libs/DF/picture.lua:251
SetFontShadow = defined @Plater/libs/DF/fw.lua:904
CreateOptionsFrame = defined @Plater/libs/DF/fw.lua:2876
SetEnvironment = defined @Plater/libs/DF/fw.lua:5364
Dispatch = defined @Plater/libs/DF/fw.lua:4412
CoreDispatch = defined @Plater/libs/DF/fw.lua:4437
MapRangeUnclamped = defined @Plater/libs/DF/math.lua:46
GetBestFontPathForLanguage = defined @Plater/libs/DF/fw.lua:3028
SortFunctions = {
}
www_icons = {
}
NewButton = defined @Plater/libs/DF/button.lua:907
PotionIDs = {
}
CreatePool = defined @Plater/libs/DF/fw.lua:5236
AddClassIconToText = defined @Plater/libs/DF/fw.lua:997
GetBezierPoint = defined @Plater/libs/DF/math.lua:72
PayloadMixin = {
}
ClassSpecIds = {
}
DefaultSecureScriptEnvironmentHandle = {
}
RemoveRealName = defined @Plater/libs/DF/fw.lua:860
GetClassTCoordsAndTexture = defined @Plater/libs/DF/fw.lua:985
FormatColor = defined @Plater/libs/DF/fw.lua:1493
SwitchCounter = 747047
AddMemberForWidget = defined @Plater/libs/DF/fw.lua:3269
InstallTemplate = defined @Plater/libs/DF/fw.lua:3172
HeaderFunctions = {
}
FlaskIDs = {
}
GetFontSize = defined @Plater/libs/DF/fw.lua:1039
IsShadowlandsWow = <fu

There has been no work done towards classic hardcore yet.

@Tercioo : it seems that the client contains a lot of merges code from retail

Is the wow classic era PTR?

Yes, the TOC is 1.14.4.