fix(button-toggle): error when check value is set via static attribute in Ivy

Fixes the button toggle throwing an error under Ivy, if its `checked` value is set via a static attribute (e.g. `<mat-button-toggle checked>`). The issue comes from the fact that in Ivy static inputs are set during creation, which means that the selection model isn't initialized yet.

Fixes #16471.

Author: crisbeto

src/material/button-toggle/button-toggle.spec.ts

@@ -845,6 +845,21 @@ describe('MatButtonToggle without forms', () => {
     expect(fixture.componentInstance.toggles.toArray()[1].checked).toBe(false);
     expect(fixture.componentInstance.toggles.toArray()[2].checked).toBe(true);
   });
+
+  it('should not throw if initial value is set during creation', () => {
+    const fixture = TestBed.createComponent(ButtonTogglesInsideButtonToggleGroupMultiple);
+
+    // In Ivy static inputs are set during creation. We simulate this by not calling
+    // `fixture.detectChanges` immediately, but getting a hold of the instance via the
+    // DebugElement and setting the value ourselves.
+    expect(() => {
+      const toggle = fixture.debugElement.query(By.css('mat-button-toggle')).componentInstance;
+      toggle.checked = true;
+      fixture.detectChanges();
+      tick();
+    }).not.toThrow();
+  });
+
 });
 
 @Component({

src/material/button-toggle/button-toggle.ts

@@ -183,7 +183,7 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
 
   /** Selected button toggles in the group. */
   get selected() {
-    const selected = this._selectionModel.selected;
+    const selected = this._selectionModel ? this._selectionModel.selected : [];
     return this.multiple ? selected : (selected[0] || null);
   }
 
@@ -276,10 +276,14 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
       (this.selected as MatButtonToggle).checked = false;
     }
 
-    if (select) {
-      this._selectionModel.select(toggle);
+    if (this._selectionModel) {
+      if (select) {
+        this._selectionModel.select(toggle);
+      } else {
+        this._selectionModel.deselect(toggle);
+      }
     } else {
-      this._selectionModel.deselect(toggle);
+      deferEvents = true;
     }
 
     // We need to defer in some cases in order to avoid "changed after checked errors", however
@@ -294,7 +298,7 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
 
   /** Checks whether a button toggle is selected. */
   _isSelected(toggle: MatButtonToggle) {
-    return this._selectionModel.isSelected(toggle);
+    return this._selectionModel && this._selectionModel.isSelected(toggle);
   }
 
   /** Determines whether a button toggle should be checked on init. */