ToolWindow Changes

trying my very first plugin dev.
the plugin has a toolwindow, and i set it an icon:

final Icon icon = IconLoader.getIcon("/images/my_icon.png");

ApplicationManager.getApplication().invokeLater(() -> {
    final ToolWindow toolWindow = ToolWindowManager.getInstance(project).registerToolWindow(
        RegisterToolWindowTask.closable(
        ":my[py]",
        icon,
        ToolWindowAnchor.BOTTOM));

the icon is displayed and all is good:

but then, after "scanning files to index", the icon changes to a dot:

at first i though it was a missing 

intellij {
    plugins "platform-images"
}

in build.gradle, and a missing

<depends>com.intellij.platform.images</depends>

in plugin.xml, but it wasn't...

also, noticed that when this happens, the following returns null: 

project.getService(MyProjectService.class);

kindly advise.