diff -u b/src/Plugin/paragraphs/Behavior/ParagraphsStylePlugin.php b/src/Plugin/paragraphs/Behavior/ParagraphsStylePlugin.php
--- b/src/Plugin/paragraphs/Behavior/ParagraphsStylePlugin.php
+++ b/src/Plugin/paragraphs/Behavior/ParagraphsStylePlugin.php
@@ -261,7 +261,7 @@
     // In case the current style is not set, fallback to the default style.
     // If default style is set to none, no style will be applied.
     $paragraph_styles = [];
-    if ($this->configuration['defaults']) {
+    if (isset($this->configuration['defaults'])) {
       foreach ($this->configuration['defaults'] as $key => $value) {
         $paragraph_styles[$key] = $paragraph->getBehaviorSetting($this->getPluginId(), $key, $this->configuration['defaults'][$key]);
       }
diff -u b/src/Tests/ParagraphsStylePluginTest.php b/src/Tests/ParagraphsStylePluginTest.php
--- b/src/Tests/ParagraphsStylePluginTest.php
+++ b/src/Tests/ParagraphsStylePluginTest.php
@@ -323,2 +323,126 @@
 
+  /**
+   * Tests the style selection plugin settings and functionality.
+   */
+  public function testMultipleGroups() {
+    // Install Paragraph Collection Test in order to have styles.
+    \Drupal::service('module_installer')->install(['paragraphs_collection_test']);
+
+    $this->addParagraphedContentType('paragraphed_test', 'paragraphs');
+    $this->loginAsAdmin([
+      'edit any paragraphed_test content',
+      'edit behavior plugin settings',
+    ]);
+    $this->drupalGet('admin/structure/paragraphs_type/add');
+
+    // Create Paragraph type with Style plugin enabled.
+    $paragraph_type = 'test_style_plugin';
+    $this->addParagraphsType($paragraph_type);
+    // Add a text field.
+    $this->fieldUIAddExistingField('admin/structure/paragraphs_type/' . $paragraph_type, 'paragraphs_text', $paragraph_type);
+    $this->drupalGet('admin/structure/paragraphs_type/' . $paragraph_type);
+    $this->assertFieldByName('behavior_plugins[style][settings][groups][]');
+    $this->assertFieldByName('behavior_plugins[style][settings][default]', '');
+    $edit = [
+      'behavior_plugins[style][enabled]' => TRUE,
+      'behavior_plugins[style][settings][groups][]' => [],
+      'behavior_plugins[style][settings][default]' => '',
+    ];
+    $this->drupalPostForm(NULL, $edit, t('Save'));
+
+    // Create paragraphed content.
+    $this->drupalGet('node/add/paragraphed_test');
+    $this->drupalPostAjaxForm(NULL, [], 'paragraphs_test_style_plugin_add_more');
+
+    // Check that a style without a style group is available.
+    $options = $this->xpath('//select[@name=:name]//option[normalize-space(text())=:text]', [
+      ':name' => 'paragraphs[0][behavior_plugins][style][style_wrapper][style]',
+      ':text' => 'Groupless',
+    ]);
+    $this->assertTrue(isset($options[0]), 'Groupless style available.');
+
+    // Check that the style options are sorted alphabetically.
+    $styles = $this->xpath('//select[contains(@id, :id)]', [':id' => 'edit-paragraphs-0-behavior-plugins-style-style']);
+    $this->assertEqual('- Default -', $styles[0]->option[0]);
+    $this->assertEqual('Bold', $styles[0]->option[1]);
+    $this->assertEqual('Groupless', $styles[0]->option[2]);
+    $this->assertEqual('Italic', $styles[0]->option[3]);
+    $this->assertEqual('Regular', $styles[0]->option[4]);
+    $this->assertEqual('Underline', $styles[0]->option[5]);
+
+    // Restrict the paragraphs type to the "Italic Test Group" style group.
+    $this->drupalGet('admin/structure/paragraphs_type/' . $paragraph_type);
+    $edit = [
+      'behavior_plugins[style][settings][groups][]' => ['Italic Test Group'],
+    ];
+    $this->drupalPostAjaxForm(NULL, $edit, 'behavior_plugins[style][settings][groups][]');
+    $edit = [
+      'behavior_plugins[style][enabled]' => TRUE,
+      'behavior_plugins[style][settings][groups][]' => ['Italic Test Group'],
+      'behavior_plugins[style][settings][defaults][italic_test_group]' => 'italic',
+    ];
+    $this->drupalPostForm(NULL, $edit, t('Save'));
+
+    $this->drupalGet('admin/structure/paragraphs_type/' . $paragraph_type);
+
+    // Check that the style without a style group is no longer available.
+    $this->drupalGet('node/add/paragraphed_test');
+    $this->drupalPostAjaxForm(NULL, [], 'paragraphs_test_style_plugin_add_more');
+    // Since Italic Group defines only two styles, assert that only they appear.
+    $styles = $this->xpath('//select[contains(@id, :id)]', [':id' => 'edit-paragraphs-0-behavior-plugins-style-style']);
+    $this->assertEqual(2, count($styles[0]->option));
+    $this->assertEqual('- Italic -', $styles[0]->option[0]);
+    $this->assertEqual('Bold', $styles[0]->option[1]);
+    $edit = [
+      'title[0][value]' => 'title_to_remember',
+      'paragraphs[0][subform][paragraphs_text][0][value]' => 'text to apply styles'
+    ];
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertRaw('paragraphs-behavior-style--italic');
+
+    // Configure Regular as a default style.
+    $this->drupalGet('admin/structure/paragraphs_type/' . $paragraph_type);
+    $edit = [
+      'behavior_plugins[style][settings][groups][]' => ['Italic Test Group', 'Regular Test Group'],
+    ];
+    $this->drupalPostAjaxForm(NULL, $edit, 'behavior_plugins[style][settings][groups][]');
+    $edit = [
+      'behavior_plugins[style][settings][groups][]' => ['Italic Test Group', 'Regular Test Group'],
+      'behavior_plugins[style][settings][defaults][regular_test_group]' => 'regular',
+    ];
+    $this->drupalPostForm(NULL, $edit, t('Save'));
+
+    $node = $this->getNodeByTitle('title_to_remember');
+    $this->drupalGet('node/' . $node->id() . '/edit');
+    // Since Italic Group defines only two styles, assert that only they appear.
+    $styles = $this->xpath('//select[contains(@name, :name)]', [':name' => 'paragraphs[0][behavior_plugins][style][style_wrapper][regular_test_group]']);
+    $this->assertEqual(2, count($styles[0]->option));
+    $this->assertEqual('- Regular -', $styles[0]->option[0]);
+    $this->assertEqual('Bold', $styles[0]->option[1]);
+    $styles = $this->xpath('//select[contains(@name, :name)]', [':name' => 'paragraphs[0][behavior_plugins][style][style_wrapper][italic_test_group]']);
+    $this->assertEqual(2, count($styles[0]->option));
+    $this->assertEqual('- Italic -', $styles[0]->option[0]);
+    $this->assertEqual('Bold', $styles[0]->option[1]);
+    $edit = [
+      'paragraphs[0][behavior_plugins][style][style_wrapper][italic_test_group]' => 'bold',
+    ];
+    $this->drupalPostForm(NULL, $edit, t('Save'));
+
+    $this->drupalGet('node/' . $node->id());
+    $this->assertRaw('paragraphs-behavior-style--regular');
+    $this->assertRaw('paragraphs-behavior-style--bold');
+
+    // Configure Regular as a default style.
+    $this->drupalGet('admin/structure/paragraphs_type/' . $paragraph_type);
+    $edit = [
+      'behavior_plugins[style][settings][groups][]' => ['Regular Test Group'],
+    ];
+    $this->drupalPostAjaxForm(NULL, $edit, 'behavior_plugins[style][settings][groups][]');
+    $edit = [
+      'behavior_plugins[style][settings][groups][]' => ['Regular Test Group'],
+      'behavior_plugins[style][settings][defaults][regular_test_group]' => 'bold',
+    ];
+    $this->drupalPostForm(NULL, $edit, t('Save'));
+  }
+
 }