Remove the clearing behavior from add_override

doc/classes/Control.xml

@@ -177,7 +177,7 @@
 			<argument index="1" name="constant" type="int">
 			</argument>
 			<description>
-				Overrides an integer constant with given [code]name[/code] in the [member theme] resource the control uses. If the [code]constant[/code] is [code]0[/code], the override is cleared and the constant from assigned [Theme] is used.
+				Overrides an integer constant with given [code]name[/code] in the [member theme] resource the control uses.
 			</description>
 		</method>
 		<method name="add_theme_font_override">
@@ -188,7 +188,7 @@
 			<argument index="1" name="font" type="Font">
 			</argument>
 			<description>
-				Overrides the font with given [code]name[/code] in the [member theme] resource the control uses. If [code]font[/code] is [code]null[/code] or invalid, the override is cleared and the font from assigned [Theme] is used.
+				Overrides the font with given [code]name[/code] in the [member theme] resource the control uses.
 			</description>
 		</method>
 		<method name="add_theme_font_size_override">
@@ -199,7 +199,7 @@
 			<argument index="1" name="font_size" type="int">
 			</argument>
 			<description>
-				Overrides the font size with given [code]name[/code] in the [member theme] resource the control uses. If [code]font_size[/code] is [code]-1[/code], the override is cleared and the font size from assigned [Theme] is used.
+				Overrides the font size with given [code]name[/code] in the [member theme] resource the control uses.
 			</description>
 		</method>
 		<method name="add_theme_icon_override">
@@ -210,7 +210,7 @@
 			<argument index="1" name="texture" type="Texture2D">
 			</argument>
 			<description>
-				Overrides the icon with given [code]name[/code] in the [member theme] resource the control uses. If [code]icon[/code] is [code]null[/code] or invalid, the override is cleared and the icon from assigned [Theme] is used.
+				Overrides the icon with given [code]name[/code] in the [member theme] resource the control uses.
 			</description>
 		</method>
 		<method name="add_theme_stylebox_override">
@@ -221,7 +221,7 @@
 			<argument index="1" name="stylebox" type="StyleBox">
 			</argument>
 			<description>
-				Overrides the [StyleBox] with given [code]name[/code] in the [member theme] resource the control uses. If [code]stylebox[/code] is empty or invalid, the override is cleared and the [StyleBox] from assigned [Theme] is used.
+				Overrides the [StyleBox] with given [code]name[/code] in the [member theme] resource the control uses.
 				[b]Example of modifying a property in a StyleBox by duplicating it:[/b]
 				[codeblocks]
 				[gdscript]

scene/gui/control.cpp

@@ -327,7 +327,6 @@ bool Control::_get(const StringName &p_name, Variant &r_ret) const {
 		r_ret = data.color_override.has(name) ? Variant(data.color_override[name]) : Variant();
 	} else if (sname.begins_with("custom_constants/")) {
 		String name = sname.get_slicec('/', 1);
-
 		r_ret = data.constant_override.has(name) ? Variant(data.constant_override[name]) : Variant();
 	} else {
 		return false;
@@ -1780,53 +1779,38 @@ Rect2 Control::get_anchorable_rect() const {
 }
 
 void Control::add_theme_icon_override(const StringName &p_name, const Ref<Texture2D> &p_icon) {
+	ERR_FAIL_COND(!p_icon.is_valid());
+
 	if (data.icon_override.has(p_name)) {
 		data.icon_override[p_name]->disconnect("changed", callable_mp(this, &Control::_override_changed));
 	}
 
-	// clear if "null" is passed instead of an icon
-	if (p_icon.is_null()) {
-		data.icon_override.erase(p_name);
-	} else {
-		data.icon_override[p_name] = p_icon;
-		if (data.icon_override[p_name].is_valid()) {
-			data.icon_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
-		}
-	}
+	data.icon_override[p_name] = p_icon;
+	data.icon_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
 	notification(NOTIFICATION_THEME_CHANGED);
 }
 
 void Control::add_theme_style_override(const StringName &p_name, const Ref<StyleBox> &p_style) {
+	ERR_FAIL_COND(!p_style.is_valid());
+
 	if (data.style_override.has(p_name)) {
 		data.style_override[p_name]->disconnect("changed", callable_mp(this, &Control::_override_changed));
 	}
 
-	// clear if "null" is passed instead of a style
-	if (p_style.is_null()) {
-		data.style_override.erase(p_name);
-	} else {
-		data.style_override[p_name] = p_style;
-		if (data.style_override[p_name].is_valid()) {
-			data.style_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
-		}
-	}
+	data.style_override[p_name] = p_style;
+	data.style_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
 	notification(NOTIFICATION_THEME_CHANGED);
 }
 
 void Control::add_theme_font_override(const StringName &p_name, const Ref<Font> &p_font) {
+	ERR_FAIL_COND(!p_font.is_valid());
+
 	if (data.font_override.has(p_name)) {
 		data.font_override[p_name]->disconnect("changed", callable_mp(this, &Control::_override_changed));
 	}
 
-	// clear if "null" is passed instead of a font
-	if (p_font.is_null()) {
-		data.font_override.erase(p_name);
-	} else {
-		data.font_override[p_name] = p_font;
-		if (data.font_override[p_name].is_valid()) {
-			data.font_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
-		}
-	}
+	data.font_override[p_name] = p_font;
+	data.font_override[p_name]->connect("changed", callable_mp(this, &Control::_override_changed), Vector<Variant>(), CONNECT_REFERENCE_COUNTED);
 	notification(NOTIFICATION_THEME_CHANGED);
 }