Add missing initializations for Node3DEditor.

Do not try to format fields with garbage values in _snap_update()
Initialize grid_enable[] before use.
Initialize previewing_camera before use.
These are all cases found live, with valgrind.
Fixes #76925

editor/plugins/node_3d_editor_plugin.cpp

@@ -8382,8 +8382,6 @@ Node3DEditor::Node3DEditor() {
 	snap_scale->set_select_all_on_focus(true);
 	snap_dialog_vbc->add_margin_child(TTR("Scale Snap (%):"), snap_scale);
 
-	_snap_update();
-
 	/* SETTINGS DIALOG */
 
 	settings_dialog = memnew(ConfirmationDialog);
@@ -8703,7 +8701,7 @@ void fragment() {
 		_load_default_preview_settings();
 		_preview_settings_changed();
 	}
-	clear(); // Make sure values are initialized.
+	clear(); // Make sure values are initialized. Will call _snap_update() for us.
 }
 Node3DEditor::~Node3DEditor() {
 	memdelete(preview_node);

editor/plugins/node_3d_editor_plugin.h

@@ -408,8 +408,8 @@ private:
 	Camera3D *previewing = nullptr;
 	Camera3D *preview = nullptr;
 
-	bool previewing_camera;
-	bool previewing_cinema;
+	bool previewing_camera = false;
+	bool previewing_cinema = false;
 	bool _is_node_locked(const Node *p_node);
 	void _preview_exited_scene();
 	void _toggle_camera_preview(bool);
@@ -586,8 +586,8 @@ private:
 	bool origin_enabled = false;
 	RID grid[3];
 	RID grid_instance[3];
-	bool grid_visible[3]; //currently visible
-	bool grid_enable[3]; //should be always visible if true
+	bool grid_visible[3] = { false, false, false }; //currently visible
+	bool grid_enable[3] = { false, false, false }; //should be always visible if true
 	bool grid_enabled = false;
 	bool grid_init_draw = false;
 	Camera3D::ProjectionType grid_camera_last_update_perspective = Camera3D::PROJECTION_PERSPECTIVE;